Ian MacGregor walks through his approach to building a phase-coherent drum mix from the ground up, starting with nominal gain staging across all channels before any processing is engaged. The workflow is sequential: establish a solid kick, snare, and overhead foundation first, then fold in every remaining element while checking its phase relationship to that core.
Do all of this in mono. Stereo spread can mask cancellation and make a phase problem sound like a vibe, so collapsing to mono makes the difference between right and wrong immediately obvious when you flip polarity. With multi-miked drums, what you're listening for is body and fullness - the correct polarity state will sound noticeably fuller and more solid, while the wrong one will sound thin or hollow.
When checking phase between two mics, both signals need to be at a comparable level - if one is much quieter, a polarity flip won't produce a clear audible difference and you'll miss the problem. This matters most on mics where the signal of interest is only incidentally captured: toms in the underheads, snare in the hi-hat mic. Bring that secondary signal up enough to actually hear the cancellation before you decide.
If flipping polarity doesn't give you a clear winner either way, the answer isn't to pick one and move on - it's to go move the mic. When Ian's underhead ribbons came up ambiguous, he walked out and repositioned them rather than reaching for the polarity switch. A phase problem that isn't cleanly resolved at the source will keep undermining the mix.