from my experience so far, you just end up kind of telling right off the bat...easier to learn if you just purposely go out of your way and underwater one and overwater one...a great use for those pesky garden males too
underwatered is limp...noodle like...even the stems flop around...can't miss it... the pot is feather light and soil separates from the container wall
overwatered are more firm but the leaves curl down...almost looks like they are sleeping for a better description...the pot is heavy and may even still drip water when held aloft
pushing a finger into the dirt usually is rewarded with a squelching sound and possibly water pooling around your finger