Every Datadog Agent reports a heartbeat called
datadog.agent.up with a status
UP. You can monitor this heartbeat across one or more hosts using a Host Monitor.
Select your host by name or tag(s). Providing a tag will monitor every host that has that tag or tag combination.
Select the no-data timeframe. If the heartbeat stops reporting for more than the number of minutes you have selected, then you will get notified.
Configure your notification option
For additional details, reference the Monitor Guide.