Might be a newb question but.

I have a buddy with a set of 15 inch slipstreams, one of them is T total ****ed.
Wants to buy a new one but isnt sure at all what size 15 it is, he hears its 15x7.5..but is not sure, there are numbers written all over the rim, just...no definitive answer to our question, how would we tell what size rim this is, if we were to take the tire off, what are we measuring by, inside lip to inside lip? or...what..help?

 

on the inside(with tire ON) there should be stickers or #s
behind the spokes. i have slipstreams and they have them there.
mine say 15x6.5