VendorMark 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 VendorMark, and the qualification matters more than the agreement. 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.

VendorMark 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.

If somebody has the primary source to hand I would rather cite it than paraphrase it.

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

kate.chem said:

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

Adding a me-too, because a thread of one person's experience is not much use. The detail I would add is minor and it is already implied above.