Come on man....do you even smoke weed??....what strains are there? Seriously??
White Widow, Jack Herer, Big Bud, ring any bells?? (ring any bells isn't a strain btw!)
How do you know what strain you are growing?? Well, when you buy seeds it lists all the strains available and you pick one. So say you buy some white widow for example.....it is pretty safe to assume that this is what your plant is going to be.
Does that make sense?