Dont have a test kit for that yet (only PH, Ammonia, Nitrate, Nitrite). I'll get one next time I go to my LFS.
I've just, out of curiosity, bought 5 gal of RO from my LFS (I wanted the 5 gal container anyway) and tested their water, and it came up exactly the same as mine (12.5 .... and presumably they're using expensive RO equipment)
Maybe it's my test kit ?