If in doubt, the first post of call would be weight. You can lose pieces off the flan, but unless obviously half a coin and therefore half the full weight it would almost always be within 5-10% window either side of nominal unless it has serious corrosion issues. Coins were worth their precious metal value, because they defined it. Sure there are exceptions, that's what knowledgeable people are for, but in the main aren't required for basic spadework.