Like a synthetic diamond, medals that have been faked can always be spotted.
The best way of doing this is to compare a possible fake to one you know is authentic. Coins and medals are ridiculously hard to counterfeit; Pascal seems to insinuate that this might be easy, but apparently it isn't. Forging notes is a doddle compared to forging coins.
In the case of named medals, the script, British official impressing or engraving is virtually impossible to reproduce precisely. Crooks are usually better advised, or find out from experience, that there are easier ways of making a few dishonest shilliings.
Faking medals has gone right back to Victorian times and has been around almost as long as government issued campaign medals.
Most are clumsy attempts and easy to spot. The clumsiest of all is to make a cast, but the fine detail of the medal and the density of inferior metal are obvious give aways. Renaming a medal to a more desirable recipient, eg A Zulu medal to one of the 24th killed at iSandlwana is an old fiddle. However, the overwritten script when compared to an original officially named piece is easy to spot and file marks and traces of the original name often remain visible underneath. A sure check is to take a micrometer and measure the diameter of the medal from 11 oclock to 5, 9 to 3 an 2 to 7. If the diameter is reduced across the first and third areas (where the naming usually lies) compared to 9 to 3 where it doesn't, this is a sure sign of renaming. All distances should be the same to within fractions of a mm.
The hardest fakes to spot are those to older medals where only the name of the recipient was impressed on the rim. For example, a naval general service medal with bar Syria, to man with a common name like say John Smith is worth about £500. By removing the bar and replacing it with a bar like say, Trafalgar, increases its value 10x. However, the sale history of all NGS medals to John Smith with bar Trafalgar are well documented going back to the 1920s. If a new one were suddenly to appear in a sale room, the question of provenance crops up and becomes important. Yes, the rivetting of the clasp may be a giveaway to a very, very experienced eye, but If its emergence can't be explained, it will generally be left alone and buyers will wait for a safer, better documented medal to come along.