Coke from the street is already cut to shit, so any information an EZ test, or something like it could yield would be whether or not there is any cocaine content whatsoever.
The melting point thing does work though, but unless you have a very controlled setting to do it in, or have access to an Amsterdam smartshop, then it isn't very practical. An easy thing you could do is try dissolving a little bit in some distilled water, and whatever is dissolved is coke, since coke is water soluble. I'm not sure whether or not other cuts are, but it might give you some idea as to the purity.
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"If we knew ourselves perfectly, we should die..."
--Albert Camus