Can other trained illusionists break through/spot/etc other people's illusions if they know to look for them? Also, if someone else has an illusion cast on them, such as for a disguise, what's it like being in one?

Also, could someone who knows the theory be able to figure out how an illusion could have been pulled off and/or call bullshit on the possibility of the phenomenon being an illusion?

Or, basically: how can an illusionist solve a mystery and catch a culprit that's supposed to be another illusionist if illusions aren't particularly permanent? (I suppose I should also ask: is it possible at all)