Most counterfeit ones are magnetic, try to check it with a magnet. But in case of non-magnetic, we can't be sure before checking all what said PetrusAscanus.
Administrateur du catalogue, référent de nombreuses nations antiques et de la Lorraine.
Catalogue administrator, numerous Antique nations and Lorraine referee.
This coin was issued with date 1871 and dates in the stars 18-71, 18-73, 18-74 or 18-75. All commons, except 18-73.
The counterfeit method consists in filing down the star with the 71, the most common. Then punch a 73, the rarest.
The star in the photo looks a 73, isn't it?. Likely a real 1871*1871 with a fake star 73. The value in this case is the bullion value, 11€.
The mint letters in the photo are SD-M. These letters are only in the 71 year. The rest of the years have DE-M. The combination SD-M with star 73 only can be in the counterfeit I say.
Referee for Spain, Iberia (ancient), Suebi Kingdom and Visigothic Kingdom