The method to check for a tampered chassis number as suggested by Daemon's mechanic is quick, reliable and somewhat scientifically sound. The original chassis punch area on any vehicle is made out of malleable metal that forms part of the original pressed steel sheet. If the area is cut and welded later, it tends to harden since the high temperatures for cutting and welding employed perform hardening comparable to regular hardening techniques used for metals. The excess carbon produced from welding electrodes tends to harden the surface of the metal at the welded joint seam and beyond. Typically, metal sheets used for car manufacturing are manipulated using brazing and not welding since welding has far higher temperatures. Brazing tends to employ much more oxy-acetylene so it produces more carbon and hence hardens the brazed area much more than even welding. If you try, you will notice that the brazed chassis number area and approximately 4 inches around it in all directions will be harder than the rest of the sheet used. If you are still unsure, you can always consult your local Anti-Vehicle Lifting Cell (AVLC), available in the largest police station in your district for laboratory testing.
Another older technique in use by car lifters was the use of beryllium copper (gun metal) to fill up the chassis' punched area and to punch in new numbers. This produces hardness in the immediate area of the chassis numbered area while the area around it is still pliable and can be bent using fingers. In either case, your local AVLC laboratory can help you determine if the vehicle is tampered or not.
Also, "nfssaad" is true when he says that he can't bend the chassis number area on his '86 Charade since thicker sheet gauges were employed to manufacture vehicles well into the mid-nineties. The chassis number of jeeps also encounter similar problems since the used sheet thickness is high and it is generally not possible to bend them with fingers alone.
On another note, you can always confirm if the vehicle is genuine by consulting the VIN number. Almost all vehicles manufactured in the nineties and beyond employ VIN numbers for verification. The chassis number is only available on the vehicle's frame while the VIN number is available on multiple areas and parts of the vehicle including wind shields, under body, seat belts, radiators etc. The exact location of the VIN numbers varies depending on the manufacturer and the particular model. A simple search on Google is generally enough to reveal where the VIN numbers are located. Typically, any vehicle has 6 or more VIN numbers in different places so tampering the VIN number is not always possible. All car manufacturers also follow a mathematical algorithm for VIN verification. For example, certain mathematical relationships between the different parts of VIN numbers on Mitsubishi jeeps add up to multiples of 11. There are hardly any cases where the tamperer takes care of such a lot of things and you can easily use this information to identify a genuine vehicle from a tampered one.
Hope this helps. Please feel free to put up any queries.
Fahad Bashir
Assistant Collector, Pakistan Customs Service