Accepted answer
Start from what "research use only" actually means, because most of the downstream questions are answered by it. It means no release testing, no pharmacovigilance, no regulatory obligation to you, and no recourse.
The diagnostic red flags, in rough order of how much they tell you: a certificate whose lot number does not match the vial; a certificate with no method section; a purity figure quoted to two decimal places with no chromatogram; a testing date that precedes the stated manufacturing date; identical certificates across nominally different lots; and "sterile filtered" offered in place of a sterility test. Each of those is a specific inference, not a vibe.
Stated carefully, lot-to-lot content variation of nine per cent between two nominally identical lots, both within a stated specification, is the single most common finding in independent testing and the least discussed. It is not fraud; it is the consequence of a fill process controlled to a tolerance rather than to a target. It is also the reason a per-lot content assay is worth more than a per-supplier reputation.
The published aggregate datasets from Janoshik, Medutest and PeptideMeter are the closest thing to a systematic evidence base in this space, and the striking pattern across all three is that identity is almost always confirmed, purity is usually acceptable, and content is where the variance lives.
The caveat is that none of this makes an unapproved product safe or lawful to use. It reduces one category of uncertainty — what is in the vial — and leaves every other category untouched.
Test the first lot from any new supplier, set your accept threshold before the result arrives, and keep the certificate with the lot number and the date in one place.