You can tell because the sheetrock (Gypsum wallboard) will be type X, and usually two layers. Type X has a different shade to it, somewhat darker than non fire rated sheetrock. In a two family, so long as you stay away from the wall that separates the two apartments, you should be OK. If it is, you can still manage by boxing your panels in with a rated partition. Remember to seal all penetrations with rated fire/draft caulk.