Perhaps the most difficult thing that the average collector faces is knowing if a stamp or cover of interest is genuine. Mexico has had it share of charlatans. Some have even been inducted into the MEPSI Hall of Fame, I cringe. But many of these people have been under the radar of most of us. Even Stanley Gibbons is credited with manufacturing fakes, and his name seems to be one of respect. So why not the Mexican counterfeiters too.
There are several keys to whether an anotado is genuine or fake. Follow the link at the left and there is a section that will discuss the overprint itself. If the Anotado overprint is OK, then the stamp is probably a genuine anotado stamp. However, there are many anotado fakes that have been manufactured with genuine anotado stamps, so one must also qualify any covers. And because a used anotado is slightly more valuable than a mint anotado, one will find genuine anotado stamps with bogus cancels affixed.
Anotados were used primarily in Mexico City. And, in fact, the stamps with anotado overprints were all sold in Mexico City, or at least the "Anotado" was imprinted there. We have recorded anotado usage in certain offices, some of which are sub-offices of Mexico City and in a couple of cases anotados were used in other main offices. The Puebla diligencias is recorded as having used anotados. But a high percentage, probably as high as 98%, would have been used in Mexico City.
But basically, if the anotado overprint has all of the proper check points, and was used in Mexico City between March 5th and April 25th of 1868, then it is probably OK. If unused, one has only the checkpoints to guide them. And there are many unused anotados.
1868 Issue of Mexico, The Great Postal