or, you could just measure the diameter of the tube. If it's 1.14" it's mil-spec and if it's 1.17" it's commercial - at least according to the instructions that came with my CTR stocks.
I don't change stocks out much, if at all, so it's a non-issue for me. The last few DPMS commercial buffer tubes I've bought were $12 each, so again, a non-issue for me.