[FireHorse]

Illusions are common enough that attempting to Disbelieve them merits its own entry in the Options list. So even if common folk don't regularly deal with them, evidently Adventurers encounter them fairly often. And that means they should also know HOW to look for those telltale signs.

The question is simply whether they DO.

So if the Player announces "I'm going to watch for signs that this dude might be an Illusion", and then later attempts to Disbelieve, the GM should award some bonus on the roll. But if the Player just attempts it cold, without any prior announcement of watching for clues, then no bonus.