APEX - The American Philatelic Expertizing Service will offer an opinion on the genuineness of philatelic material at a moderate cost. The rate is significantly less for APS members. APEX utilizes the services of more than 140 specialists (as of 2008) and high-tech equipment.

PSE  -  Professional Stamp Experts - Advantages of using the PSE service include, among other things, a prestigious board of philatelic experts, state-of-the-art technology, and a quick turn-around. Their opinions are accepted by all of the major auction firms. They offer grading and slabbing services for a fee.

PF - The Philatelic Foundation - The Philatelic Foundation Certificate, the PFC, has long been an industry standard and the PF has one of the finest reference collections available. 

William Weiss Stamp Expertizing Service - I am adding this expertizing service to the list because it fills a very important niche in the authentication of U.S. stamps. Many stamps fall into a gray area in which their value is less than the cost of authentication. This is particularly true for stamps that are not exceptionally well-centered or that may have small faults but are still often the object of tampering. Further, although many Washington Franklin coils may be well-centered and sound, there are fakes never-the-less; perhaps a trimmed sheet stamp, or more likely a stamp made from imperforate stock. Although these stamps may be relatively inexpensive, they are often faked for precisely that reason. It is not cost-effective  to authenticate the stamp and the faker has a good chance of getting away with his crime. Other areas of concern are re-perforation, cleaned cancels and re-gumming. Mr. Weiss has lowered the bar considerably regarding the cost of authenticating these stamps. Might I add that even in the event that the cost of verifying the stamp as genuine exceeds the value of the stamp, having a reference collection of both authentic and tampered stamps, fully identified as such, will prove of far greater value in the long  run than the cost of authenticating a handful of stamps.

If you buy or sell stamps on the Internet, particularly on eBay, you should check out this article by eBay user philcomp: Buying Higher Value ($100 and up) US Stamps on eBay . All I can add is that "higher value" might as well start at $10 or even less, for you will find doctored stamps in that price-range as well. 

Of course, I would also recommend checking out individual Washington Franklin stamps on this site. Many of the pitfalls for individual WF stamps are detailed, by Scott number, just enter the Scott number of the stamp in question into the search box at the top of the page.