Assuming you don't have any Vis sources prepared, how do you adjudicate if a PC mage is combing an area (say a valley or wood or a town) and using vis sensing spells to try to find new sources or just randomly coalesced vis?
Do you randomly roll to see if any is there, just decide by fiat, have the mage make a Perception + Awareness roll or what?
Has this come up in your saga?
Base it on the Personal Vis Source Virtue (page 47 , ArM 05) , as YSMV on how much Vis is available.
Work out how much Vim vis could be extracted (page 94) as an equivalent to found vis , also.
If the player gets a particularly high stress roll on searching , allow them to identify extra sources ,
which can be harvested at a later time , rather than a huge amount of vis in one go (if that suits better).
See the Presence power on page 103 , RoP: Magic.
Perception + Magic Lore with no active spells , Perception + Awareness would be if the magus has a Sight-based spell allowing detection.
There should be some Ease Factor numbers for the Magic Lore roll , but the location escapes me atm.
I've found a good rule of thumb for vis sources is roughly 3-4 pawns/magus/year for an average covenant. Drop that down to 2 if you want to run vis-poor, or go higher if you want them to be rolling in the stuff.
Regardless of what the initial boons/hooks and build points say, aiming for something in this vicinity over time should stabilise their vis economy. Remember, though, that this includes vis from trade or other sources as well as found in the wild.
If the covenant isn't bringing in that amount, use their role to determine whether they found a renewable vis source. If they roll well, have them find some non-renewable vis as well. Everyone likes a little reward.
If the covenant has already hit their stride with vis income, you can mix things up a bit. Either they don't find anything, or they find a vis source that has complications (claimed by another covenant, protected by a difficult beastie, corrupted, etc). Of course, there's nothing stopping you from throwing the occasional one of these at the players before they hit their vis stride.
I'd personally shy away from basing their success purely on a die roll. Vis isn't something that there's an unlimited amount of - in a vis poor game a successful per+magic lore roll may tell them 'nope, you've tapped out all available sources.'
The other fun thing about vis is it's seasonal, and in many cases it requires some interesting processes to harvest. Simply walking around looking for it may not be enough: you need to talk to the locals, keep an ear out, be in the right season, be under the right stars, etc. So even if you say 'nope' one day, you are perfectly justified saying 'actually... yes!' later.
Because of the intricicies that can involve vis hunting, I'd normally not allow a per+awareness with spot-vis spells to work. I'd instead say a person doing that gets a +1 on their per+magic lore roll, since it makes verification easier. That or it lets you send your mundane magic-lore-monkey out looking for vis under that enchantment and you can stay at home in your lab. Mmmmm, lab.