Mataxes wrote:"Short-term" (almost always damaging) illusions cannot be 'sensed'. The target must decide at the time whether they believe it or not. Believe it, and the magic affects you. Disbelieve, and it doesn't. The drawback is that if you disbelieve and it's real, you open yourself up and the effects are greater (because your MD is considered 2 for the purposes of determining successes, this means the spell will almost certainly score at least one success more than it otherwise would have).
The question at hand, which Undying has addressed to a large degree, is when to prompt players for the choice to disbelieve. Obviously you don't come right out and say "Illusionist!" if they don't already know. But slow players may never catch on.
A fine line, I guess, so perhaps there is no perfect solution but one tailored to a particular group.