NeuroNate 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.

Dr.EndoIndy said:

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

Shipping damage protocol for vendor vetting orders: if your package arrives with visible damage, temperature excursion, or broken vials:

• Photograph EVERYTHING before discarding packaging
• Note the temperature of the cold pack (liquid = possibly too warm)
• Contact the vendor within 24 hours with photos
• Request a replacement shipment or refund
• Do NOT use potentially compromised medication

Reputable vendors will replace damaged shipments without argument. If they push back, that's a red flag about the vendor.

RetaRick_CA said:

I disagree that testing history is decisive.

There is a second half to this that has not been said yet. Take it one variable at a time. Almost every unanswerable question in these threads is unanswerable because three things changed in the same fortnight, and no amount of subsequent argument can untangle them after the fact.

A narrower follow-up, since the general answer is now clear:

What a vetting checklist should actually contain, as opposed to a list of things that are easy to fake?

Closing the loop on my own question.

Went back through my own correspondence with an earlier supplier and the warning signs were all there in the replies, weeks before the quality changed. I was reading the product and not the behaviour.