Hi and welcome!
A handful of vendors sit in the top tier or two when it comes to testing. The process is pretty simple: a vial or two goes off to the lab, you cover the cost of whichever test you're after, and back comes a COA (Certificate of Analysis - lab report). Mass/purity (M/P) and Endo (endotoxins) are the tests you'll see most often.
Testing solo gets pricey fast. So if keeping costs down matters to you, there are a few routes: put together a group or hop into one so the expense gets divided, buy through a group buy (a bulk purchase shared by many people that usually comes with a discount) which typically bundles testing in, or simply hold out until somebody who bought the same product from the same batch submits it for testing and posts the results for everyone.
When safety is the priority, my suggestion is to filter while you're reconstituting your pep.
