If you know strains, or at least visual differences in a nug of weed then you can tell what kind of effects it will have. Is it sour diesel or durban poison? Than you know you're getting sativa, and you know you can smoke it in more social settings. Hashplant? Kush? Get ready to be put to sleep in the most relaxing way. Then there are hybrids that can "lean" in either direction.

If buds are big, fat, and dark green, chances are it's indica dominant. Indicas also tend to smell skunky and/or spicier than sativas. If they're sativa they'll generally be smaller, longer buds and a lighter green. They will smell sweet or fruity. Hybrids will display characteristics of both.

There are always exceptions though. I've heard stories of certain landrace sativas that look like dirt schwag, yet give you a fearsome psychedelic high that lasts 5+ hours.