I think just by reading the labels on the ROMs. I'm pretty sure the boards themselves are the same.
Edit: Actually, I'm not even sure the regular 2P ROMs are different... maybe someone can confirm? My 2P Track Pak board has different ROMs than the 3P Track Pak, but IIRC the 2P non-Track Pak cabs actually had 3 players, but controls were only hooked to two of them (unless all the 2P cabs I played just had 3P boards installed).
DogP