Visually there is no way to determine if you are looking at a 305, 307, 327, or 350. The only sure way is to get the engine code/suffix that is stamped on the land area just forward of the right hand (passenger) cylinder bank. The links provided by the others here are decoders which will tell you what size the engine block "should" be.
One method to tell if you are looking at a 2 bolt or 4 bolt engine without dropping the oil pan is to see if there is a small pipe plug on the block behind the water pump. If the plug is there you more than likely have a 4 bolt block.
