You can also determine which printing it is by the paper. #6 were printed in London by De La Rue & Co on Hard medium high quality paper. It is somewhat translucent.
The early #7s were printed in Richmond Va on the same kind of paper, sent from England along with the plates. Later #7s were printed on local paper which was thin to thicker, but totally opaque.
Nothing's ever simple is it?
Just for fun (and who says stamp collectors don't know how to have fun!) I've scanned my three CSA 5¢ stamps on lined paper. The Jeff Davis on the left is a #6 while the other two are #7s. See the difference in printing quality? The left side stamp is a much clearer impression. It was printed in London. The paper has a slight translucence - you can just see the lines on the paper behind it showing through.
The centre stamp is a local printing (Archer & Daly, Richmond VA) on the London paper. It is really translucent, the background lines are very clear. It has a bit of a waxy feel (not that I go around stroking my stamps all day). The third stamp is a Richmond printing on opaque local paper.

Just looking for one of these in a deep blue colour, and that page in the album will be complete!