OK, time to clarify some things.
The presence of any significant percentage of regrind can be detected quite simply because every moulding cycle will manifest in some degradation (polymer chain scision), which in turn will manifest in lower MFI (Melt Flow Index).
To check a sample’s MFI you don’t need large samples, the method is not complicated, the equipment is not that sophisticated, not that expensive and not that rare. If it’s a recurring situation you can simply buy a basic MFI analyser; otherwise contact a lab that provides this (basic) service. This will tell you with reasonable confidence whether the article was made from pure virgin material or includes a significant percentage of regrind (please remember that some degradation should be expected in any but an ideal moulding process, so the MFI of a moulded sample will almost never be exactly the same as that of a 100% virgin raw material).
What’s complicated is determining the exact percentage of regrind. That would need calibrating first, ie moulding samples at different known regrind percentages and finding out their respective MFIs. Of course, in order to be a valid test method a whole host of issues would need to be accounted for (as per usual when developing a test method) and some statistical rigor would be required. Another complication is material having gone through more than one regrind cycle (a natural result in a process that uses in-line regrinding).
Basic MFI is a crude tool; it’s not that accurate and the resolution is limited. I’d normally recommend it only for saying whether regrind has been used or not. It’s good because it’s simple, quick and not too expensive. However, there are additional, more sophisticated methods such as DSC and DMA, that can tell you much more about the polymer’s molecular weight (which will tell you about degradation), and are more accurate. Those methods do require more expensive equipment and some expertise in running the tests and interpreting the results; however, they are normally available from any large lab dealing with polymer testing. I didn’t check with UL (no affiliation here) but I guess they offer such testing.