Every five (5) seconds, each node in a cluster should be signaling that it's up, running, and updating. The last heartbeat is the most recent moment when a node has reported itself being active in this way. If, however, a given node has not updated its last heartbeat for more than 5 minutes, a warning icon will display on your Admin tab > High Availability page. More information about Monitoring for Unresponsive Nodes is available HERE.
Typically, when an issue arises with one of your servers, the state of the server on the HA page will remain as Running. However, when a server heartbeat has not been updated for a long interval of time, this will be displayed in red with a warning notice.
If you'd like to know if all of your servers are currently running, use the REST API calls and ping any node that you think may be experiencing a problem. If you'd like to know when was the last heartbeat for a given server, take a look at your artifactory_server tables.