Open Administration view> Agents & Groups> Agents
v12.3 agent will display the 'Last Check' in the AWI because they are using keep-alive messages which updated the value in the DB.
Starting on 21.0.11 this checking is not being performed anymore for 21+ agents. TLS agents use native ping-pong messages. Only legacy agents (12.3 or earlier) will correctly display the 'Last Check'