A certificate of analysis is the only document that tells you what is actually in a vial. It is worth knowing how to read one, because the gap between a genuine third-party report and a formatted PDF with a logo on it is where most of the problems in this market live.
Four things matter. Identity — usually confirmed by mass spectrometry, which should report an observed molecular weight matching the theoretical mass for that sequence. Purity — an HPLC chromatogram with the main peak integrated as a percentage of total area; the number quoted on the label should be traceable to that chromatogram, not asserted separately. Net peptide content, which accounts for residual water and counter-ions and is often meaningfully lower than the stated purity. And the lot number, which has to match the vial in your hand.
A report that gives a purity figure without showing the chromatogram is asking to be taken on trust. So is one with no lot number, no test date, and no identifiable laboratory. Independent testing means the laboratory is a separate entity from the supplier — a name and an address you can check.