Put on some tight, closed headphones. Crank em up. Put a mic in front of a speaker cabinet, up close. Crank up the gain. Now put another mic up to the speaker cabinet on a boom stand, cranked up, sideways where you can swivel it in and out close to far. Now, while listening, move the swivel mic in and out. You will hear it get louder, and then a weird phaising effect, then it will get thin and nasally and almost gone and then back thourgh the effect until its louder again. That is the two mics moving in and out of [phase, and its VERY easy to hear and notice. When they are in phase, the signal is in effect doubled up, and will seem loudest when in phase, thinnest when out, and screwey when anything in between.
Once you hear it and know what to listen for, you can do it on drums and whatever too.