wait a min...
We ha da thread asking exactly this question a few days ago...
USE THE SEARCH ENGINE!!! (Not even the search engine, scroll through some pages dude!)
To answer your question, the structure should give you a pretty good idea, based purely on senses of smell, and sight - if it's kinda spindly, not particularly dense, very lime green, and it smells very very fruity (particularly lemony or mangoey), there's a good chance it's a pure/sativa dom strain.
If it's very dense and smells of spicey hashish, or deep and earthy, and slightly darker, then you're looking at a nice indica heavy/pure indica strain.
However, most ganja you get these days is heavily hybridized - as crosses are more social (not too stoney, or too THC heavy) and more efficient for the commercial grower to get a mix in the genes.