Witnesses can retire by setting their key to STM1111111111111111111111111111111114T1Anm. (NULL key) For example, teamsteem is a not an active witness anymore, you can see from his signing key, it's the NULL key: https://steemd.com/@teamsteem

Also, most of the witness pages mark inactive witnesses, e.g: https://steemd.com/witnesses. Inactive witnesses have a special strikethrough effect in steemd interface.

Also, checking witnesses' latest posts also might point the active/inactive status. Example: https://steemit.com/palnet/@helpie/helpie-update-january-2020-witness-is-disabled

Cheers!