Google both of them and you will be able to tell by looking at the magazine bases.
The 7.62x39, has a flat, upward slopeing base plate and the 7.62x45, has a straight across base plate. The pics on surplusrifle.com will show you what I'm saying, far better than I can tell you.
Very nice rifles by the way but they are much more tedious to break down for cleaning than an SKS.