"and how can i tell when a bowl (i've only ever smoked out of a pipe/bowl before) is cashed?"- You'll know because you wont get anymore smoke out of it and its pure ashes.

"is there any kinda technique or etiquette i should be aware of if i'm puffing from a bowl, like how i light it?" - Light it from the sides at first as not to burn all your weed up right away. But when your looking for the very last shit you can burn it however you like.

"if i decide to buy my own piece, anything i should know about taking care of it other than cleaning oout the resin?"- Just clean it out regularly after scraping the resin and just be gentle.