not sure where you get the idea you arent allowed to have supplements tested. SMRTL, UCLA are private labs, they are not owned or controlled by USADA. If I want to walk into SMRTL and request testing of something I can.. If im prepared to pay for it.
detection of a substance in a raw product and detection of a substance excreted or metabolised would be pretty different to be honest.. Testing the supplement you would look for the actual substance, but testing in an anti doping sense you may be testing for a metabolite rather than the actual substance.
but anyway, you dont need to. There are groups like Informed-Choice.org and NSF who batch test tens of hundreds of supplements and certify them as free of prohibited substances.
Anything carrying these logos has been third party tested
interestingly, the UFC's own supplement sponsor "performance inspired" (which happen to be part funded by WME/IMG) have all their products tested and certfied by informed-choice.