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 (APEX)
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 Inc. (PSAG)
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
A more expansive printed list is found in the INTRODUCTION section of the Scott Catalogspublished by Amos Media, often available through your local library.