What do you storyguides describe when your pc's encounter a character who is "tainted with evil"?
Just general "bad vibes" or is it something more concrete? (I don't envision that the flaw goes so far as to wilt flowers or anything that nasty.) Do you generally require an awareness check for PC's to notice the flaw?
I would try to use negative, slightly monstrous, images to describe the character. So, for example, I might think of goblin-like images (pointed teeth, yellowed sickly skin, foul breath that smells of decay, and rough, cold skin) when describing what is actually a normal man that has the Tainted With Evil Flaw. If you've read it, think of how the Heathcliff character is described in Wuthering Heights.
And maybe apply a penalty if the tainted character attempts to positively influence someone.
Certainly he doesn't have supernatural powers that cause flowers to wilt, but you can stick him in an environment where there are wilted flowers, and then draw the players attention to that via your description.
No not usually (unless the PCs have some flaws that make them quite unaware of other people). However, they might want to make some kind of test (or cast a spell) to confirm that the character is not some kind of supernatural beastie. I guess that's a good measure of success. If the players think you might be describing an evil supernatural creature, then you have successfully described a character who has the Tainted with Evil Flaw.
It's difficult to "enforce" such reactions onto PC's- similar to other Flaws like meeting someone with Curse of Venus, for instance- "You are really attracted to this person", or "You feel this person is oozing with evil"- meh, some Players can RP this better than others, but it's still awkward at best.
If I know the Players, I try to push their buttons less directly with my depiction of the NPC, the words/images I paint, but, imo, Virtues/Flaws are not something that should necessarily be identifiable as if on a checklist of possible options.
If the Players/PC's merely understand that this person "bothers" them, that they feel they shouldn't trust them for an unspecified reason, then that's enough to get the job done.
I would "cheat" after a few minutes of talking: making the players roll Folk Ken, and tell them there's something untrustworthy and sinister about the character. Maybe something slightly inhuman, and that he clearly look at them as one would look upon his next meal (if they think he's a demon-blooded or magical creature , good to them).
Thus, they would be better inclined to trust this ("skills don't lie"), and act accordingly. After all, they succeeded on their check.
Just because someone's tainted with evil doesn't mean it's blatant. Could even be the opposite. Think "Dorian Grey".
I hear you Belladonna. However, I guess I was speaking more about the blantant-ness of the flaw. If a person is not trying to hide their nature, what about his/her "corruption" do storyguides describe to the players upon the encounter.
I disagree.
What you describe is a fonction of character. Someone could very well be tainted with evil, and still be a good guy.
If it wasn't blatant, it wouldn't be a flaw 
Yep, I got that wrong.
ancoraimparoancoraimparo...I just keep chanting my mantra.
Good point, but I'll quibble on your choice of words-
If it wasn't "effective", it wouldn't be a flaw.
That is, it doesn't have to be obvious, or consciously noticed, to be effective. No signpost is needed, nor does it have to be identified as such. It just has to produce the desired end result.
If Players (not PC's) realize that it's the Flaw "Tainted with Evil", then they may or may not attribute negative aspects to the character. But if they, the players, "feel" there's something wrong, then their PC's will act accordingly, and the Flaw is doing its job.
The character can, indeed, be evil, or this flaw could be more like "Judged Unfairly". It's easier to manipulate the Players to the desired end than to tell them all to "RP misjudging this character" and then expect them to do so. Regardless of the best intentions, the final product will be different when that OOC info lurks in the back of the mind.
Yup, you're right.
But then again, we agree on this, it's just a poor choice of words on my part.
After thinking about this a bit more, I have a couple of suggestions. One is, have the evil character do creepy things. If your player is talking to a guy and meal worms crawl out of the guy's mouth, your player will probably get the "tainted with evil" idea. It doesn't have to be THAT obvious, but you get the idea. "Show don't tell."
More subtly, the tainted character could have a bad habit that he gets away with but gets the people around him in trouble. If your players spend any time around this guy, they will get blamed for petty theft and bar room brawls and the tainted character always seems to get away with it.
All of these things depend upon how evil you want this guy to be. He could be a vampire or he could just be some freak that no one likes.
I Hope that helps!
Of course, the best is when you use a Player's meta-tendencies against them.
Have a sympathetic NPC who you over-describe in heavy-handed terms, such that he's "clearly" got the "Tainted with Evil" flaw "unfairly", and the Players know it, and then dismiss that NPC as "the evidence" grows and points to a different source for the "actual" evil. Maybe he seems a bit incompetent, milquetoast, sadly unlucky, generally ineffective, whatever. But it turns out he was the BBEG after all! (ala Lost Boys, etc.)
I see it as a bit less blatant, an aura of unease that slowly builds over exposure. So, a charismatic and charming tainted individual may initially come across as the "attractive bad boy" or "predatory graceful" type, but as more time is spent around that person feelings of discomfort and unease grow, perhaps the observers feel jumpy or itchy or get that feeling of being watched and after more time around him they end up jumping at shadows or feeling like bugs are crawling on them or they get paranoid, all building to the point they just can't stand to be around the guy. All of this gets magnified by less savory types having the flaw.
I'm also inclined to allow for a mitigation of the effect by time spent away from the tainted individual. It never goes away but drops back to a previous level of agitation depending upon how long the separation. e.g. I've got a Tainted with Evil Redcap in my Saga, who's also a Gossip and Busybody. He was raised by infernalists but rescued by his parens and is generally a nice guy who is honestly concerned for others and he also takes his job of gathering news quite seriously. He knows he makes people uncomfortable so he tends to not spend much time with any one group of people and being constantly on the move. I sort of see him being a bit like Karl Kolchak as played by Darren McGavin in the original Kolchak:the Nightstalker.
YMMV of course.
One of my favorite all-time anti-heroes. But Kolchak was just a schlep, one who, thru both lack of charm and force of habit, gave a bad first impression, but occasionally (rarely?), thru sincerity and enthusiasm, won some over. He made folk uncomfortable because he was a little off, both socially and with his beliefs, not because "an air of corruption", but because he got too personal and acted like he'd do anything to get a story, which he would.
Which doesn't seem to match the phrase "Others feel very ill at ease around you...", which is as immediate and obvious as Offensive to Animals. It's like really bad breath- it might well grate worse on a person over time, but that doesn't mean it's not really bad right away.
The thing about Flaws like Tainted with Evil or Offensive to Animals (or The Gift, which combines those two effects, only without the "evil" aspect) is that they work immediately and consistently across the mundane population, or at least they do "as written". Other Flaws such as "Judged Unfairly" (which seems closer to the esteemed Mr. Kolchak) might work a bit more over time, but all trigger early on in any encounter, if not immediately.
In theory one could have a similar Flaw which works only over time, however the wise Storyguide would have to first question exactly how much of a handicap that is, depending on how often they expect the character to be interested in longer-term relationships. A Lab Rat with Tainted With Evil is rarely affected, the same with some "Evil Grows on You Over Time" flaw might never actually trigger any real effect with any but an apprentice.