The Lens of Truth can also be used the see "invisible walls". To do this you use the flags 16-32, I think, and you put the flags on the unwalkable tiles, that look similar to normally walkable tiles. Then you set the secret combo(s) which correspond to the flag(s) that you used. The secret combo(s) should be made to look like walls, that way the lens will allow you to see walls that are invisible.
IMPORTANT: Don't put flags in the room with your invisible walls that would allow for the wall to be seen, unless you want the wall to be visible. And don't select the enemies/secret option for that room as it will have the same effect.