I personally don't think that a high presence is able to be turned off and on at will - without some acting skill or the like. There are numerous people that just draw others to them without being particularly good-looking (though, I don't think that has ever hurt), so that when they just walked into the room, you knew and when they talked to you, you found yourself nodding* - even if you don't agree with their point of view. It isn't a conscious act, just how they are.
So, unless the individual is actively trying to keep from being noticed / blend into the background socially, they are constantly projecting their high presence and everyone they meet is affected. (Whether good or bad.) How they hide it is up to the GM/system, but acting or even shadowing might be good ways to go.
As for how to detect another individuals "stat mods" I think the best you can get is sharply defined definitions like: very strong, extremely stupid, beautiful, slow as a slug in the winter, etc... That is the best way to go with that, I think.
*I am not saying that they change your mind completely, just an immediate affect of being in their presence. After the fact, unless they successfully got you to agree to their point of view, you are likely to go back to you own.