hank_denver said:

Prices dramatically below market are the strongest single signal, and the reason is arithmetic rather than suspicion.

No disagreement with hank_denver. One condition attached. The pattern that distinguishes a bad batch from an exit is behaviour rather than product. A bad batch comes with communication, a reshipment offer and a batch number. An exit comes with slower replies, pressure toward less reversible payment methods, sudden discounting, and the same reassurance repeated without any new information. The product tells you less than the correspondence does.

hank_denver said:

Prices dramatically below market are the strongest single signal, and the reason is arithmetic rather than suspicion.

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. 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.

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

PharmD_Rodriguez said:

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

This is my experience too, for whatever a second data point is worth. The detail I would add is minor and it is already implied above.