Answer by phg for How to check if a computer have already joined a domain with Samba

Use net ads testjoin:

# net ads testjoin -kJoin is OK

→ This box is joined. Now remove it from the domain and testjoin returns -1:

 # net ads leave -kDeleted account for 'DEADBEEF' in realm 'EXAMPLE.COM'# net ads testjoin -kJoin to domain is not valid: NT code 0xfffffff6# echo $?255