I've recently started playing The Binding of Isaac, and I've noticed that some walls can be bombed to reveal a secret room containing coins, or items. The problem is, I can't tell which ones can be destroyed and which ones can't, leading me to place (read: waste) bombs on random walls.
So, is there a specific way to tell which ones will lead to an extra room?
There is no foolproof way to find the secret room (except with some of the items you can pick up, like x-ray glasses), but 99 times out of 100 you can find it with one method and a little luck.
Most secret rooms are contained in between at least 3 other rooms. All you have to do is look on your minimap for 5 rooms that are all joined in a square "U" shape (3 on the bottom, and 2 for the sides). Bombing in the middle of the wall that faces towards the gap in the U from any of the 3 rooms that will share walls with the room will lead you in.
You can then bomb your way out into another room if you so choose. This can be useful in getting into the treasure room with no keys if the secret room borders the treasure room.
Some maps have more than 1 "U" shape in them, in this case you're just going to have to guess and hope to get it on your first go.
Some maps won't have even 1 U shape, in this case, the room will probably be in bordered by 2 other rooms, so look for a set of rooms in a square "r" shape and hope that the one you picked is the right one.
The super secret room (Wrath of the Lamb DLC only) is found in a different way. The super secret room will NEVER border more than one room (except very rarely the regular secret room).
To find the super secret room, look for any walls where if there was a room on the other side, it would have no other entrances, similar to the treasure rooms and shops.
Unlike the secret room, the map (or the tarot card "The World", or any similar items) won't show the super secret room. X-ray glasses and the Spelunker hat will still work however.
To shorten the answer Mr. Smooth gave I'll say this:
The secret room is usually the room that's most surrounded by other rooms. Sometimes this is not correct and you'll have to go to the place that's next most surrounded.
EDIT: Actually, usually the secret room is in a U shape as mentioned in other answers. Going with the above method is decent but not optimal.
I haven't found any kind of pattern yet to find these bombable walls reliably yet, but you can use recharging items like Doctors Remote or Bob's Rotten Head to find them without wasting too many bombs. Usually it will be useful to look at the map for pointers. An empty square enclosed by four rooms will often indicate a secret room.
Actually I noticed that in one of the levels the brick walls surrounding the room had a question mark like pattern on one wall, I bombed there and lo and behold there was a secret room. I don't know if this is consistent with all the levels, but I'll be looking out for it and will let you know whether or not it is consistent.
In rooms that possibly adjoin the hidden room, try to defeat enemies that explode (such as the large gray fly) near the middle of a suspect wall. It's a free test, regardless of whether it works.
If you find an Explosive Diarrhea tarot card, save it if you can for a room with a possible bomb-able wall. Start close to the target, use the card, then move away. A less-controllable variation would be with the Anarchist's Cookbook, which causes bombs to drop in random locations, possibly suiting you or not.
Alternately, if you find the Dr. Fetus item, you can fling bombs everywhere. Tends to make finding secret rooms easy!
