The Defense Manpower Data Center's (DMDC) Military Verification service can be used to verify if someone is currently serving in the military.
Information needed includes the Social Security Number and other personal information and the service will indicate that the Department either does not possess information regarding the individual or that the individual is in the military (in a signed printable letter format containing the Department of Defense seal).
See https://www.dfas.mil/garnishment/verifyservice.html