Well, I'm not an expert or anything, but here are the basic ways that I know to distinguish weed, etc...
1. There are exceptions, but usually if the weed is light green, it's sativa dominant, and if it's darker green, it's usually indica dominant. Sativas produce more of a trippy head high, while indicas give you more of a body high. Personally, I prefer sativa.
2. If you have a small bud, and go to break it up, and it's FILLED with seeds, that pisses me off, because the quality of the weed isn't as good, and it also means that most of the weight of the weed is actually in seeds, which means less weed for me.
3. If the buds are small, but they break up great, and don't have alot of seeds in them, you should feel satisfied.
4. If the buds are small, but break up amazing, and have no seeds, you're in luck. That's sinsemillia. The weed devoted more time in producing resin with THC in hopes to catch pollen, so you get more THC. And, more weed, since your bud is pure weed, and no seeds.
5. If you're saving seeds to plant, save the large dark ones. The small frail white, and/or light green seeds are usually immature, and will break between your fingers if you apply pressure. There's no point in keeping those seeds.