Dr.RenalNash said:

The pattern that distinguishes a bad batch from an exit is behaviour rather than product.

That is correct as far as it goes, and here is where it stops going. 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.

Correct me if the detail matters more than I have assumed.

Dr.RenalNash said:

The pattern that distinguishes a bad batch from an exit is behaviour rather than 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.

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.

Ask again with the specifics and you will get a better answer than this one.

Dr.GastroMayo said:

The useful checklist is about verifiability rather than presentation.

This is my experience too, for whatever a second data point is worth.