Some of the 350 blocks will be obvious, insomuch as the certain castings are guaranteed to only be 350's.
Look for the number located in front of the cylinder head on the passenger's side (on most passenger car engines, this will be right behind the alternator). The boss sticks out slightly, and the flat part on top will have the number stamped on it. You may have to do a lot of cleaning to get to this number. This will decode a lot of information about the engine.
For the last three digits of the engine casting, you'll have to look at the side of the block, below the headers. You should be able to back-check limited information on the engine with that number.
Finally, most of the late-model small blocks will have the displacement cast into the side of the engine in liters, as opposed to a last-three-digit code. So, 5.0 will be a 305, and 5.7 will be a 350.