Fournier forgeries exist on all values and types of the Navigation & Commerce (Tablet) keytype issues of 1892.  These are very common, and unfortunately, are not worth very much.  I have seen many advertised as the genuine issue (with a price to match).  The major distinguishing characteristic is in the cornucopia on the upper right side of the stamp.  In the genuine issue, the lowermost fruit on the left side is seperate and distinct from the rim of the cornucopia. In the forgery, the fruit looks like a bump on the rim. 

Fournier himself realized this distinguishing characteristic on this issue.  He would "cancel" the stamps with a phony cancel, being very careful to place the cancel to hide this area.  He also did the same when he applied overprints or surcharges to these issues.

A secondary characteristic of these forgeries is that the hand holding the staff looks like a blob on the forgeries, where it has distinct fingers on the originals.  If you have an original and a forgery and compare them, the forgeries stand out as looking generally indistinct in the details.

The forgeries exist on every one of the 491 basic issues, the remainder surcharges, The Offices in China, and many other varieties.  As these forgeries were very common, many old albums have copies in them.  The age of a stamp collection is not an indicator that the stamps are genuine.  In fact, I have found that the opposite is more likely.

A good scan is necessary when selling these items.  As there is a good market for these stamps, a good scan is very necessary to avoid getting fake instead of the real thing.  Without a good scan, I will not purchase one of these unless I know and trust the seller.

Fournier was a very prolific printer.  This was not the only issue he counterfeited.  He produced fakes of hundreds of stamps that were issued up until the 1940's.  He counterfeited stamps from almost every country that issued stamps.  Some of the originals he counterfeited catalogue in the hundreds of dollars, but many catalog in the 20-30 cent range.  There are several websites that catalog and illustrate his forgeries,. 

If you do find that you have these in your sale lots, do not despair.  There are many specialty collectors that collect these forgeries.  I have sold many over the past years, but I have always listed them as to what they really were.  They are not worth the many dollars that the genuine are, but they are still a few cents each.  In a couple of cases, I have got more than the originals were worth. 

These items are very common, but as they are easy to detect, you should be able to avoid being taken in by these.