There's a few rooms that have an intended solution that will always spawn with them. Aquarium room always comes with an invis potion, a chasm room with 1 chest always has a levitation potion, a poison gas room always has a purity potion, a magic fire room always has a frost potion.
There's a few others but the full list escapes me at the moment.
Knowing this you can often deduce what an unidentified potion you picked up on the level is.