Well, 2 things, lidocaine can be obtained easilly very cheap in some 'smart' shops, here in  this EU country. Second, my experience with coke cutted with lidocaine, is that when you cook it, the paste take much longer to become solid again. Very pure coke will become solid again after a very short period of heating even when you still heaten it.
Another indication for purety is when you cook it, the stuff has other colors, a part is darker yellow than other.