Claims are free; evidence has a cost to fabricate. When comparing sellers of the same or similar compounds, the most reliable signal is not what a page says about itself, but what it makes checkable by someone who owes it nothing.
What to look for
A named, contactable testing lab. "Independently tested" without naming the lab tells you nothing that can be verified. A named lab with its own published contact information or verification tool can be checked directly, without relying on the seller's word.
Lot-level certificates, not product-level claims. A single purity figure quoted for a whole product line is a much weaker signal than a certificate tied to the specific lot the vial came from. See lot codes and traceability.
Test scope stated explicitly. A seller that lists exactly which of identity, purity, endotoxin, and sterility were run (and doesn't run all of them on every lot) is giving you more real information than one that gestures at "full testing" without specifics. See reading a certificate of analysis.
Consistent naming and CAS numbers. A listing that gives a CAS number for a single compound, and whose sequence and formula match public reference data for that number, is easier to verify than one that only gives a trade name. See peptide nomenclature and CAS numbers.
A stated, specific return and defect policy. Vague or contradictory return language across a site's own pages is a sign the policy hasn't been thought through, which is worth noticing even where it isn't directly about testing.
Historical lots that stay published. A seller that keeps old certificates available after a lot sells out is making a longer-term commitment to checkability than one that only shows current stock.
What isn't strong evidence on its own
A high purity number by itself, without a named lab or a lot-specific certificate behind it, is not verifiable. See why purity percentages are not comparable across labs for why the number alone, out of context, doesn't mean what it appears to mean. Testimonials, star ratings, and general assurances of quality are not evidence of what was actually tested; they're marketing, and marketing is exactly what a certificate-based comparison is meant to route around.
How this catalog is meant to be checked
Products in this catalog link to the current lot's certificate and lab-verification link from their library entry and product page; a batch without a lab report is clearly marked before purchase instead, and older lots stay listed on lab results after they sell out. We don't run that testing ourselves; an independent lab does, and our lab results page points to that lab's own record, not just our copy of it. None of this is meant to be taken on trust: use the same checklist above here, too.
Have a vial or lot code to check?
Look up a lot