Right when the level starts look around the room. If there is a secret room in the beginning room, you'll see a hole where the room is for a split second before it turns into just a wall. Dunno if that's intentional or not, but it works. Also, check your map for rooms that are connected. If on your map you see two rooms with a blank space next to em, bomb it. Its most likely a secret room. Also if you're in a room with bombing enemies, let them bomb the walls before you kill em.
One kind of secret room is only ever touching one other room. And I'm pretty sure it can't be any kind of special room.
The other kind of secret room is typically touching 3 or 4 other rooms. If there aren't any places like that, it can be placed so it's touching two rooms. Note that this room is placed after the other kind of secret room, which can count as one of the 3 rooms touching it.
The treasure map will show the location of the second kind of secret room.
There's always one of each kind of secret room on a floor.
Secret rooms are buggy on XL/Curse of the Labyrinth floors. They may not follow the rules properly, and sometimes they act like normal rooms. There's also a chance that sections of the map will be connected only by a secret room, so be careful if you're low on bombs; it's possible to trap yourself on the wrong side and be unable to continue.
I'm a member of the Midnight Crew I'm a night owl, and a wise bird too