Actually, that IS a good question. I've never really thought about it. (just trusted my source, and never disappointed).

Scarlet Sky (above) says smoke it, and if you get high, it's good. That works. Can you ask for a "sample", or purchase a small amount and try it first? Then buy some more.

Once you find a good source, you can keep going back. But NEVER fink on your source or tell anyone who is not also going to be a customer too.

Good Luck!