It could be difficult to locate any lab willing to test already mixed peptides because the mixing step itself introduces many unknowns such as the quality of the liquid used, breakdown over time, temperature, and acidity. Even if a lab agreed, the result would only reflect the average across every vial that went into the pool. It would not confirm the strength of any single vial, only the overall average strength and purity. For example, if nine vials held the expected amount and one held nothing, the average would still look acceptable even though one vial was empty. The same problem would appear if two separate kits were combined and one kit was completely empty; the test would show half strength and there would be no way to tell which kit was bad.
For peptides that stay stable once frozen after mixing, what if someone reconstituted every vial in a kit or two, drew the same measured amount from each, combined those draws into one new vial, and sent that for testing of identity and amount or concentration. From there they could split the tested mix into smaller vials holding roughly a month’s supply and freeze them. This would confirm that every vial in the kit actually contained the right peptide and would give the exact strength of that particular group of randomly filled vials. All vials would be covered by the test and the user would know the true strength being used. One possible issue is that the lab might report whole numbers only with no digits after the decimal. Another is simply locating a capable lab, since some places will not take liquid samples at all.