Yeah, make sure it's fucking mescaline. That's a good start. If it's in the form of peyote, you're cool.
If your dealer says he's got "synthetic mescaline" or anything of the sort, be *very* careful. People have been known to sell the 2-CT group as synthetic mescaline as well as 2-CB (which, incidentally, is worth doing as well, as I understand it). I'd personally marquis test what you had to see if it came up as 2-CB (green) or 2-CT (Erowid says it turns salmon in color -- I can't personally confirm it, but I'm sure someone else can).
And if you indeed have *real* mescaline, stock up. I hear it's absolutely amazing.