In a recent game we began wondering if the arcane sight spell lets you see illusions, glyphs of warding, and other kinds of magical traps. We agreed that the spell would reveal the auras from glyphs, symbols, and most other magical traps, but not see an aura around the illusion of a door, floor, or creature.
It’s correct that the arcane sight spell won’t automatically allow you to look right through an otherwise opaque figment, such as an illusory door, floor, or wall. Any active illusion, however, has a magical aura that divination spells such as detect magic or arcane sight can reveal. In the case of arcane sight, you know immediately if anything you can see has a magical aura, and you know what that aura’s power is (as explained in the detect magic spell description). You also immediately know the aura’s location. If what you’re looking at happens to be a figment, you do not know it’s a figment. You can, however, make a Spellcraft check (making the check doesn’t require an action from you) to determine the aura’s school. If the check succeeds, you know that the aura is from the illusion school, but you cannot tell its subschool (it could be a figment, glamer, pattern, phantasm, or shadow).
Looking at an illusion with arcane sight counts as interacting with it, however, and if the illusion in question allows a saving throw to disbelieve, you can immediately make a saving throw. If you have identified the aura’s school as an illusion, you have grounds to find the illusion’s reality suspicious, and you get a +4 bonus on the saving throw (since you know it’s some kind of illusion). If you make a successful saving throw to disbelieve a figment or phantasm, then you can see through it, although the figment or phantasm remains visible as a faint outline (see the discussion of the illusion school in Chapter 10 of the PH).
