well, you should just be able to hear it. you'll hear a type of flanging sound and the sound won't be as full as it should be. If you have two or more mics on something, a good thing to do is switch the phase (or polarity or whatever the hell you want to call it) on that channel and see which sounds better. this may rectify the problem you're hearing, but it's also good to just move the mic around until you get rid of the phasing issue and get the sound you want.