The honest answer is that a small first order tested independently beats any amount of reading, and it costs less than most people expect.
Ask what happens if a result comes back outside specification. The answer is informative regardless of what it is, and the question itself tells you something about how it lands.
The independent testing services this community references — Janoshik, Medutest, PeptideMeter and VendorInvestigate — publish results that can be searched by supplier, compound and date, which makes prior lots checkable.
Nothing here is medical advice, and research-use compounds are not approved for human use in any jurisdiction.
Small first order, independent submission, then scale. In that order.