Hey Dynamite, first things first in seeing if what you are buying is anygood.
Look at it, if it looks nice it probably is, if it looks harsh, it probably is. Thats the first and easiest one.
Look for POINTLESS stalks and seeds, if there are lots not attached to any bud, its really low quality.
Soon you will be telling the difference between nice bud and bad bud in looks.
The smell is also another factor, if it fucking stinks the place out then its a good thing.

Over all though, the proof is in the pudding. Toke it up, if it was shit don't go back to that guy who sold it to you, or ask him if he has the same stuff next time, so you know what you're getting, or not getting.

Last thing, watch out for wet/damp weed. If it wet, its not good enough. You lose money buy buying wet/damp weed because once it drys you lost all the weight of the moisture.
NEVER GUY GRIT WEED. If its grit (chew some), do not pay for it at all costs.

Good luck, happy toking.