So, to find a secret room, it's pretty simple once you know what to do. What do you have to do? Look for X's in the 'decor' tiles in each room, and then blow them up with a bomb!
As pictured here:













Or a sneakier one here:













These (to my knowledge), can appear in any normal fight room on floors 1-12. This means event, blacksmith, tavern, shop, boss, and hatch rooms can't have secret rooms in them.

Oh, and for extra usefulness, go ahead and try to hit a few enemies with that bomb you placed to open the room.