If you are in the EU, there are several harm reduction organizations that test psychoactive substance. CheckIt in Austria, Energy Control in Spain, DIMS in the Netherldans. You can find info those organizations in a EMCDDA publication. I don't have the linky handy but I can help, if you attempts will be unsuccessful.
As for US, DEA has complicated things. You can get ratios of active substance but that data is close to useless since no quantities can be given. Read more at http://www.ecstasydata.org/about_data.php
Why would you need a standard? With access to a mass spectra library such as NIST, or just any publication showing the mass spectra, you can easily tell appart MDMA from other related substances and adulterants.
As for the OP.. By purity do you mean % or you mean if its mixed with something else? In the second case, TLC is enough, in the first case, TLC coupled with photospectrometer (Both quite cheap) can tell you with a decent accuracy how pure your substance is. If you want to detect even trace substances, then GC-MS or LC-MS will be necessary, but for most purposes, TLC is just fine.
No... unfortunately, only the police and certain labs can do a purity test (toxicology). All a reagent test can tell you is whether or not your drugs contain MDMA... can't tell you what other adulterants or contaminants might be in it.