Hi I'm looking at creating a demon hunter type of character and an obvious flaw I've noticed is that he doesn't really have a way of seeing through or detecting the presence of demonic deception, he can make powerful wards and blow up all but the most powerful of demons in a couple of rounds if they face him directly but he'll be completely susceptible to their deception. So far the only real way I've seen to do it is with Ars Notoria, but that requires a major virtue + 250 levels of abilities at character creation for just a single desired aspect and I'm not particularly willing to make that investment. What other options are available?
There are work-arounds.
The Virtue Sense Holiness and Unholiness allows you to detect demons, sometimes. It won't detect deceptions directly, but it can help detect deceivers.
Working around the lies themselves benefits from detective work. The demons can only lie: they can't rob you of logic or faith. Furthermore, demons have a poor grasp of virtuous behavior and no grasp of virtuous motivation, so they only frame things in terms of vice. If a known virtuous person is suddenly behaving in a strangely and overtly sinful way, there's something wrong.
Demons cannot deceive angels; if you have a guardian angel, they will not be deceived by a demonic deception that would pass any mundane or magical perception. Angels are not always communicative, though, and are often cryptic. They will always work in your best interest, though, and may give you clues or warnings or good advice.
Yeah I took a look at Sense Holiness/Unholiness but the ease factor of 15 for using it on a single target was a bit harsh.
Guardian angels also feel a little... DMPCish and I'm sure a lot of story guides wouldn't enjoy an angel too much, thought they are probably the best option if I don't want to dedicate my entire character creation to ars notoria or something similar.
Second Sight can help you against some shapeshifted demons or illusions, but in general, detecting demons is very hard.
Intentionally so.
Yep. In fact the impossibility of telling when a demon is lying allows for "detective story" plots which are of course a little hard in Ars where Sight of the Transparent Motive can work out who did it in one spell. Though if Agatha Chritie teaches us anything it is that everyone can have a motive to the the murderer...
Still for a Demon Hunter take Sense Holy/Unholy, though I think that is normally associated with the Infernal or Divine so might not work as well in magic auras. I seem to recall Aura penalties on the interaction table apply to Supernatural Abilities?
cj x
Indeed.
Indeed. Though Ofcourse the Divine doesn't care.
Can second sight see through demonic illusions? I thought that was more for faerie/Hermetic trickery.
My only issue with sense holiness/unholiness is the ridiculous 15 ease factor for individuals, plus if I'm reading it correctly you have to penetrate magic resistance on top of that.
There is the Major Story Flaw Plagued by Supernatural Entity (ArM5 p.58), which can be had with an angel trying to save you.
This is often quite adequate for a demon hunter magus, especially if the angel is ArM5 p.198 Seferiel. This angel will strive to have you tread the straight and narrow, but also to protect you from the finer webs of hell.
If interested, make sure to hash it out in detail with your troupe or Alpha-SG.
Cheers
Hmm, that could work for an over-confident/prideful character
I've seen it used to notice invisible demons.
Check with your troupe/SG before you invest though 
While it doesn't help with demonic lies.... And I know that Hermetic magic cannot detect a demon's illusions.... Can Hermetic magic dispel the illusions?
Logic and Faith.
Oh, yes. Dispelling known illusions is leaps and bounds easier than figuring out that they're illusions to begin with, assuming you have the right spell (bear in mind that the PeVi spells for ending supernatural effects are Realm-specific, so you'll need a whole separate spell for ending Infernal effects than you would for Magic).
It is by design that demons and their mischief cannot be easily detected. It is specifically designed to prevent PCs to simply blast spells left and right until it pings a demon, so they can shift to DEO.
Basically, you hunter needs to be clever: using ward across a hallway to prevent a demon to go through, knowledge of infernal lore to know weakness or compulsion of demons. Some cannot stand the smells of certain spices (cinnamon), or myrrhe, some needs to collect every single poppy seeds. It makes more challenging to be an inconspicuous demon hunter, but it also makes for more interesting stories than just: roll a die for XYZ spells/abilities, then blast the beast.
You already mentionned that your character can plow through demons like butter. It his schticks, good. But if he can also detect demon easily, ST will only throw once in a blue moon a demon in front of him because he can so easily detect and dispose of it. And for you, after blasting a demon in two rounds a few times, that will just be boring.
My take is: don't try to make the perfect demon hunter, because there won't be interesting stories to tell. On contrary, build it with relevant weakness that demons can play with him as well. When he will beat them the victory will be that more rewarding.
I am sure we all build the perfect , and I usually do not have fun very long afterwards. It is much more interesting to build a character who is aspiring to be excellent at one task than to have one who is already at that level. As you PC strive to reach this level of perfection, he will need to find a Cult to initiate specific virtue, possibly convert to Holy Magic, acquires relic and so on. That's golden for a ST, plenty of opportunity for adventure. On the other hand, Torquemada the Fifth, armed with Faith, Relic and the right cocktail of spells is much more boring: he only aspire to hunt demons, then the stories will be repetitive and not so fun after the first one.
generally I have noticed a number of items which routinely auto-cast spells for destroying demons or demonic illusions... since it is easier to wipe them out than to detect them.
... How does an item auto-attack something it can't detect? I mean, the item's detection ability falls under the same rules as Hermetic magic in general...
Unfortunately, you can design a area effect (Room, Structure...) base on Demon's Eternal Oblivion, casted automatically every round (trigger event: when this spell stops being in effect).
Since it is an area of effect, it does not need to have a target or be targeted.
Sure, it increases the base magnitude of the spell, but since it is relatively easy to build high Penetration in magical item, you can have a very simple way of destroying demons without seeing them.
As ST, for the sake of interesting stories, I would frown upon such items, but we are entering the touchy area of House rules.
In terms of being able to excel rules-wise (skipping the valid story implications or problems other players have correctly pointed out), there are a couple ways:
If you are a magus, learn Demon's Eternal Oblivion at level 1 and buff up your penetration. Try to cast it at eye contact range. Take either subtle/quiet magic, or master it so you don't need voices or gestures. Casting it on a non-demon will not harm or affect it. A demon, however, will be marginally weakened and likely seriously annoyed. Look for the 'what was that?' reaction. Assume your ST will have this power backfire on you, as even if your magic is subtle enough, you are staring at people rather oddly.
My personal favorite: A low-level Rego Corpus effect that fluffls the target's hair for a moment, as if in a minor breeze. It is designed to be cast forecelessly. The idea is that if the spell fails to work, it's because your target has magic resistance of some kind. This won't detect how they have it (could be a faerie, someone inside an Aegis, a Hermetic Mage, angel, or demon), but it does show they are resistant. Demons are immune to Intelligo magic, but not necessarily other kinds.
You could also simply ask people to join you in prayer. My understanding is that in ArM5, demons are too proud to enter a church or pray. They might disguise themselves as churchgoer or even a priest, but refuse to actually go through the steps. This of course will have a low rate of success, as many good people are not Christian, but it could be a starting point.
Again, as other players have pointed out, it wouldn't be much fun to excel at demon-noticing. The above are techniques that can be used to make your character more skilled, but do work with your ST to make sure they help the story be more fun.
Not necessarily. One of the important ideas to embrace is that Ars Magica at its heart is about player character magi who can do great things.
So, the game is working as intended if the player character magi are able to do stuff and kick arse (especially if it requires some clever use of spells).