Home signals (signals with multiple heads) are interlocking signals. These govern the entrance to interlockings. These are easy to distinguish because of the multiple heads.
Approach signals however, usually have only one signal head, and operate in a similar manner to automatic signals. They are usually seen before or when approaching interlockings or terminals. The condition of approach signals are affected by the conditions set by the tower at interlockings, but are not absolute (they can be set to show 'Stop' but they can be passed at slow speed or 'keyed by'). Approach signals are controlled off the same lever and can the control length can span over multiple signal blocks.
Basically, if you ride from the head-head enough you can tell which ones approach signals; usually by seeing two or three consecutive stop signals, very close together when there is no train inside the signal block.
Check out www.nycsubway.org/tech/signals it explains it all pretty thoroughly.