The link below details not only some tips in spotting imposters, but details where doing such is an actual crime. Mostly it's about impersonating officers and medals of valor, but there are other limits along with some states which make posing as a veteran a crime.
IMO, it should be a felony to impersonate being a veteran regardless of rank or otherwise lie about service in the US military including time in service or type of discharge.
Learn about stolen valor and similar military crimes. If you’ve been accused, defend yourself with help from an experienced military lawyer.
www.jordanucmjlaw.com