COA_Karl said:

Eight months with one supplier, then a change in how they communicate that I noticed before I noticed anything about the product.

Agreeing with COA_Karl, and the qualification matters more than the agreement. 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.

COA_Karl said:

Eight months with one supplier, then a change in how they communicate that I noticed before I noticed anything about the product.

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.

Taking the question as asked, rather than the general version of it. 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.

PurityPaulOR said:

The useful checklist is about verifiability rather than presentation.

This matches mine closely enough to be worth saying so. 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.