There are four principal services in the United States where you can submit stamps or covers for an opinion about the item’s genuineness. Remember that they only offer opinions.
American Philatelic Expertizing Service
Provides opinions on stamps and covers from around the world. Searchable database of prior certificates.
The Philatelic Foundation
Provides expertizing services for all stamps and covers. Their site allows you to search over 500,000 previously issued certificates
Philatelic Stamp Authentication and Grading
PSAG will authenticate U.S., Canadian and some other stamps. See their website for details.
Professional Stamp Experts
PSE grades U.S. material and some foreign items. See their website for details