All fuels have markers (isotopes) that have slight variations from one fuel source to another. If I have a verified source, I can tell if a sample came from that source or not. It is not cheap to do, but it is pretty reliable. Also, remember that canisters filled by the manufacturer use dried propane. Most (probably all) of the people re-filling are using bulk propane that is not dried, so interior tank corrosion occurs (which is also usually the cause of the tank rupture) and I can characterise that. Also, I microscopically examine the valve and I can see slight marks that are indicative of filling. If you refill, and have an incident, you definitely don't want me as the Forensic Engineer working on your case because I will find out the truth. In most cases, the dots are pretty easy to connect on all three.
The last guy not only had consusive issues, but burns as well. After my analysis, not only did a "suit" against the manufacturer get vacated, his homeowners policy denied the claim. It was messy for the guy and he could no longer work either. It was his attorney and the insurance company attorney who hired me jointly.