Anyone interested in Henry III voided long cross pennies may like to visit my website, www.henry3.com Rob
I know of no hard and fast rules. There are 'museum reproductions' of a lot of hammered coins and they tend to look suspiciously too good. Buy from a known supplier or have a look in Spink to ensure it looks similar. Is the price in a reasonable ball park for the coin? Does the metal look right?
Just thought I'd post this, my latest Edward I penny Class 5a, London mint, minted between 1289 and 1291, pellet on breast. (N. 1028, S. 1399) I know London coins are common as dirt, but Class 5a and 5b are listed as scarce, so I just had to get this one when I saw it.