i just started at 50mg/day on thursday and i'm lucky enough to not have felt a thing (: we'll see what happens when we start going up though

Unfortunately, this particular cadence doesn't always happen, so you'll need to determine what is the most "at rest" chord in some other way, sometimes. Almost always, however, the cadence will help you.

What distinguishes A minor from C major is what chord feels the most resolved. If you have a dominant seventh chord moving to another (non-dominant) chord one fourth above it/one fifth below it, that's a good indication that the second chord is the tonic of the key/mode.

A scale is a collection of notes; keys and modes are implementations of those collections in the context of a piece.

"Keys" generally just refer to aeolian and ionian ("in the key of C"); "modes" are everything else—even outside of the usual seven ("this piece is in D dorian b2" or whatever).