PharmHunterJen said:

Six orders from the same supplier over eight months, every one independently tested, and the consistency is the part worth reporting rather than any…

PharmHunterJen has the substance of this right. The condition it depends on is worth stating. The useful checklist is about verifiability rather than presentation. Independent test results commissioned by buyers rather than by the seller, consistency across multiple batches over time rather than one flattering report, a physical address and a company registration that resolve, batch and lot numbers on the actual labelling, and a shipping practice that matches what the material needs. Everything a good-looking website provides is cheap to fake; none of the items above are.

PharmHunterJen said:

Six orders from the same supplier over eight months, every one independently tested, and the consistency is the part worth reporting rather than any…

I disagree that testing history is decisive. It tells you what a supplier did when they were being watched. Continuity of behaviour under stress — a late shipment, a failed test, a complaint — is more predictive than any run of good results.

Answering the narrow version, because the broad one does not have a single answer. Prices dramatically below market are the strongest single signal, and the reason is arithmetic rather than suspicion. Synthesis, testing, and cold-chain shipping have floors. A price well under the floor means something was skipped, and the two things that get skipped are testing and content.

LarryQC_SD said:

The useful checklist is about verifiability rather than presentation.

Same position here, arrived at the long way round. One addition: an unstated method makes a report close to uninterpretable. "99% pure" without the technique, the standard and the conditions is a claim, not a measurement.