have a look at it under a weak 3x magnifier or whatever you can get your hands on, that way you can get a close look at the bud and see what the trichomes(the thc filled glands on the bud)are lookin like. because after all thats what your looking for at the end of the day, good ole thc.
usually i think thats a pretty good indicator, if the buds are nice and crystally, they usually do a good job.

edit: obviously there are alot of other factors that come into it, how they were cured, smell ect, but thats the first thing i look for.