I'm guessing you'd need a lab. take 'em to HM customs?
you'd need something to determine the isotopic percentage of the elements in there (assuming they still contained tobacco, i assume different tobaccos would have slightly different isotopic percentages), or if they were really cr@ppy (i.e. something in there that's not tobacco), you'd need an NMR machine (cost around £500,000), mass spectrometer (at leat £50,000), elemental anaysis (dunno about the cost), and IR (again, probably at least £50,000) wouldn't hurt either, to determine what IS in there instead of the tobacco (or as an impurity).
Not to mention a lab to enable you to extract the impurities.
Unless there's some easier way of which i'm unaware...