To verify whether a person is in the military right now, search the Official Department of Defense
Servicemembers Civil Relief Act (SCRA) website (if you get a message saying there is a problem with the web site's security, continue on anyway). This requires a social security number or date of birth. If you don't have this information using a pulic records database such as TLO or Accurint. Alternatively, you can place an order with the Servicemembers Civil Relief Act Centralized Verification Service or Recoursa. You can also ask for an Affidavit of Military Status from the Department of Defense through MilitarySearch.org.
To see if someone has ever served in the military, try searching Military.com and/or the locators available to subscribers at ArmyTimes.com, NavyTimes.com, AirForceTimes.com and MarineTimes.com.
U.S. Military Personnel Records (for service in the 20th or 21st century) are maintained by the National Personnel Record Center (NPRC). You can request copies of records by filing an SF-180 with the NPRC. The NPRC posts SF-180 forms and information on the filing procedure.
The NPRC makes military records available to the public for military personnel who served and were discharged, retired or died while in the service, prior to 1946. The NPRC posts information on how to retrieve these records.
Alternatively, you can get service records from the NPRC faster by calling Military Information Enterprises (314-423-0860) or sending them an email. They have staff onsite at the NPRC and will retrieve information for "veterans; their next of kin, if deceased; or for attorneys (by court order)."
Historical military records are available online through Fold3.
For more information, see Military Service Records and Unit Histories: A Guide to Locating Sources by the Congressional Research Service and/or Finding Service Records by the Library of Congress, which covers "Individual Personnel Files," "Official Unit Records and Published Unit Histories," and "Unit Morning Reports and Rosters."
Finding military personnel:If you are simply trying to find someone currently or previously in the military, in addition to the sources discussed above, see FAQ - How to find US Military personnel and/or the links for the U.S. military and other other countries.