If you haven't used the Deployment Monitor app yet, you should take a look. It can tell you when you're receiving less data than expected, and you can configure alerting.
You could do this as a very broad, slow, naive search:
* | stats last(_time) as _time by host, source | sort _time, host, source
But, looking at the _internal index for metrics would probably be best, which is what the Deployment Monitor does.
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