The majority of weed grown/purchased is actually a hybrid of sativa and indica. 100% Sativa's are pretty rare. 100% indicas are a bit more common but still you're most likley getting something that is a cross between the two.

I don't know anyone who can look at a dried bud and say "Elemenatry! My Dear Watson. That bud is 33.456% Sativa and 66.544% indica". As far as color is concerned, I have noticed that the really dark buds, especially the purples and blacks, do tend to be indicas but there are lightwer colored indicas too. The way a bud looks has as much to do with how its grown and cured as it does the strain. So just looking isn't the best way to tell.

The best way to tell is actually by the high. If you were to smoke a pure indica, something like Hindu Kush, and the next day smoke a pure sativa, somehting like Haze (most Hazes are actually sativas) you would definitely be able to tell the difference. The indica would make you want to zone out on the couch and the sativa would give you a more energetic high. Again the problem is that most are hybrids with characteristics of both. But its generally safe to say that if you want to go out and do something after smoking the weed is predominately sativa. If you just want to chill its got more indica in it.