Every product has a cost and a market. That sounds obvious, but it is the single most useful line of defence I have, and I have never seen it used by a buyer.
A price below the market is information, not a discount
When a quote comes in under the market, the instinct is to feel lucky. It should feel like a question instead. Something has to give: either the margin, or the quality, or the truthfulness of the person quoting. Only one of those three is being given away, and it is not usually the margin.
There is a version of this where the supplier is over-estimating what the product costs them, and they are simply being generous. That happens. But the burden of explanation sits with them, and "we can do it cheaper" has never yet turned out to be an explanation.
The transaction that taught me to check
I acted once as purchasing agent on a meat product from another country. The price was under market, and that put me on alert - because I had seen it before, and because a product cannot cost less than it costs.
So I went through the paperwork looking for the problem. Business registration, certificates, the lot. I could not find a fault in any of it. The documents were not sloppy; they were convincing.
What finally settled it was information from the bank. The company did not exist as the documents described it. Everything the seller had shown me was genuine - and none of it was about them. That is the part I still think about: the paperwork was real, and it was about something else entirely.
What I do instead now
I run the number backwards before I read anything else. Landed cost per kilogram, or per drum, on my side, given what I know about the product. Then the quote goes against that floor.
Comfortably above it means room to negotiate, and a normal counterparty. Below it means the conversation has already told me something, and the useful question is no longer "is this a good price" but "what is different about this product".
What I am not telling you to do
There is no checklist here, and I am not going to write one. A checklist is a tool for verifying a supplier you have already decided about. The judgement I am describing is different, and it is not transferable: it is a way of knowing which question is worth asking next, and that only comes from having been on the wrong end of it.
What I can tell you is the shape of the problem. If a quote does not survive a backwards calculation from your own cost, then the material you receive is the thing to think about, not the paperwork that came with it.