Hi and welcome!
There are a few vendors that are considered in the top tier or two for testing. Basically, you send in a vial or two to the lab, pay for what kind of testing you want, and they provide you a COA (Certificate of Analysis - lab report). The most common tests are mass/purity (M/P) and Endo (endotoxins).
They're quite expensive to test on your own, so if you want to save on costs, you can organize or join a group to test to split the costs, purchase from a group buy (bulk buy from many people, get price discounts) which likely includes testing, or wait for someone to have bought the same product from the same batch who sent it in for testing and shares the results with everyone.
If you're focused on safety, I recommend filtering during reconstitution of your pep.
