In-ear earbuds. I'm not a drummer myself, but I have a buddy who is a pretty talented drummer. usually without the earbuds, Yeah, you'd get lost without them during live shows. Look at videos of live drummers- nearly all of them have in-ears.

and not all drummers play the same tempos during live shows, it's pretty much impossible. for example, I've seen lamb of god three times. chris alder is an insanely talented drummer that's been around for a long time- but he still gets off beat sometimes, and when he does, the others pick up or lower their guitars/bass to match him and his rhythm. it's cool to witness.

I think it really just comes down to muscle memory and how well you work with and can communicate with your band mates. paying attention the the music, following everyone else's lead, you know?