Well as was clearly stated by several other posters earlier, proving the metal content doesn't really do much for us. I'd leave that to GW, they could probably conjure up the right lawsuits and equipment -atomic absorption spectroscopy and a few other expensive techniques would be the only foolproof i think, since pewter and white metal are complex alloys and it would be hard to spot any real differences utilizing only sulfuric acid and the naked eye (ouch!). Actually, the smell, appearance and taste (taste carefully) of lead pewter is very characteristic and would serve better as proof on our level than any hobby chemistry experiments i think.
Actually, now that I come to think of it, lead is supposed to be prone to oxidation by acetic acid to produce Pb(II) acetate which is then supposedly reduced back to black elemental Pb in air (the lead comb). If this applies to pewter, however, I don't know. I would also be very careful if you would try this, chiefly because of the acid and because the black stuff is poisonous.
Always fun to rant about chemistry
Now, I think GW should be contacted on this one. Especially if the miniatures contain lead, since they cannot be produced by GW in that case (hey, maybe we should try that vinegar stuff anyway!).
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