How do you tell tobacco types? Newbie help

Basically, what I'm saying is, for the n00b (me) who purchase tobacco online, how do we tell what type a tobacco is (cavendish, latkia, perique, burley, etc.)?

I often check the website I'm ordering from, but Boswell's doesn't often specify, and other sites may not, either. I'd like to know ahead of time so I can stop mixing blends in one pipe, and dedicate my sole briar and my corncobs to specific blends.

Also, how do you decide which route to go when a blend is compromised of more than one tobacco type?

Thanks - Kegan