look for the crystals and pay attention to the formation of the buds... too much leafage means it sucks... and check the seeds, if they're undeveloped, they should be green, not white, otherwise it's shit, if they're developed, they should be fully developed, squeeze one really hard to see if it crushes... but actually the only way to really know is to smoke it. ive had shitty looking pot that was the bomb, and awesome looking shit that sucked my ass