Illusionists are the hardest to deal with because there's so much "is it a believable illusion" debate. If I saw a cell-shaded scooby-doo illusion I'd be looking for the special effects but if it was a brown great dane I'd just wonder where it came from.
We adjucated it a bit by using Knowledge:bestiary or nature vs. opponent's rolls to realize it was "wrong" prior to any interaction tests. Of course wrong didn't always mean illusion since it could mean a poorly done Alter Self/disguise for creatures or look like a trap (i.e. illusionary wall looks weird so it could look like paper mache).
Of course the trick to good illusions is as much content as accuracy. All to often the illusionist goes for a dragon or beholder, which are too weird for people to take at face value. My illusionist/summoner sorceror would summon a few critters and send them into the fight in the first round and the next cast an illusion of twice as many of the same creatures being summoned in as protection for him and the cleric (usually 30' away to be outside blast radii).
Other good illusions are scouts/horns of the local constabulary, powerful spells that are *just* off target (disintigrate is a good one since it just makes a tree disappear), spiritual weapon/mord's sword and summoning large common creatures (bears, bulls, boar, etc).
When you go to cast an illusion, go for the simplest, smallest, most stationary, and probable illusion that can do the job. Odds are it's also the most familiar and the hardest for you to screw up.
Oh, and when in doubt and you need to illusion in people, use your family. No DM in their right mind can say you can't cast a decent illusion of your mother, father, siblings, and childhood friends.