It is hard to detect many of the fakes and copies listed these days. One easy one is the 1776 "FF" Mexico city 8 reales.

 To start , all the 8 reales of that time were minted with a Mint Mark and Assayers Marks. Look on the reverse at about  8 o'clock ,left of the base of the left hand Pillar. Mexico City's mark was an "M" with a small "o" above it. The Assayers' mark is at about the 10 o'clock place, near the top of the same pillar.

 The Assayer in 1776 for Mexico City was "FM" and only FM , any "FF" coins are fakes. In 1777-79 the new and old assayers overlaped , and again 1783-84, but not in 1776.

 Aside from my own testing on a number of these coins , Krause does not list them and Jose A. Vicenti in "Catalogo General De La Moneda Espanola" also shows none minted.

Keep your eyes open!