Determine the type of paper the stamp was printed on. If it was printed on the "hard white" paper it is either a National or Continental printing. If it was printed on the "soft porous" paper, it is an American printing. 

If it is an American Bank Note printing, compare the area near the arabesque with the illustrations above to determine whether it is Scott 182 or Scott 206.

If it is either a National or Continental Bank Note printing, check for the "secret" mark. If it has the "secret" mark it is the Continental Bank Note stamp,  Scott 156.

If the stamp was printed on the "hard white" paper and does not have the "secret mark", it was printed by the National Bank Note Company. If the stamp has a grill, it may be Scott 134. If it does not have a grill it is Scott 145. Care must be taken when authenticating the grill. Many fake grills have been added over the years in an attempt to increase the value of the stamp. Scott 134 is common enough that it may not be necessary to certify all copies, but nicely centered, fault-free, and particularly unused stamps should be certified.

If you are unsure of the type of paper used or, more likely, of the secret mark, you MUST assume that the stamp is the more common variety. 

Watch also for  removal of the secret mark by scraping, for re-perforation (to fake a more well-centered stamp), for re-gumming (a major problem with the Bank Notes), and even for bleaching of the cancellation (to remove the cancel). 

We include the "Special" printings in this identification guide merely for completeness. Only 388 copies, total, of Scott 167 and 192 were sold, and they were never issued for postal use. All were printed without gum. We occasionally see uncertified copies of these stamps offered for sale at ridiculously low prices. You can rest assured that the stamp being offered is not genuine. These stamps are so rare, only 70 copies are known,  they rarely come up for sale, except in the sale of a major U.S. Collection.