I couldn't tell you how to test the purity of a sample of heroin, but Le Junk's cocaine purification thread is a handy way of gauging the purity of the cocaine you were sold (although this method sounds like it requires more than a sample.)
http://www.bluelight.org/vb/threads/317605-cocaine-purification-the-absolute-final-thread
Ah, thanks for that reference.
Le Junk's thread is edifying and his techniques seem rather serviceable for small-sized quantities of cocaine (say, 5 grams or less). But it appears not to be as workable with the purifying of poundage. Indeed, with the quantum I have in mind the procedure would possibly prove itself exceedingly tedious and torturously time-consuming. It seems that one wanting to purify a kilogram, say, would necessarily have to divide it up and do it piecemeal.
Moreover, that thread, while greatly informative and thoroughgoing, pertains to the purification of the substance. I am not so interested in purifying a sample of cocaine as I am in simply determining its level of purity. That is, I would like to know the existing purity rather than to augment it.
Let's envisage that hypothetical Mr. Hong Gildong, once more for the sake of illustration. Hong Gildong has a rendezvous with a heroin deealer in some nondescript café in Bangkok. After an exchange of pleasantries, a hushed tête-à-tête, and a brief and inconspicuous exchange of dollars and dope, Hong Gildong exists the café and heads back to his modest pied-à-terre, forthwith. There, he inspects his brick of heroin by sight, consistency, and having himself a rail or two. The stuff's primo, he concludes. But Gildong wants to know how stepped on his H is, so he knows how much to step on it himself.
How might the curious Hong Gildong determine the percentage of heroin contained in his powder? He wants to cut it to 60%, but can only do so if the starting purity is known, as the ratio he uses to adulterate the heroin and achieve a precise percentage of purity is contingent on how adulterated the drug already is.
I should note that I'm absolutely no bagman or peddler, but just an inquisitive individual fascinated in the trafficking of commodities on the black market.