check rules analyzes and pinpoints misconfigurations in rule configuration files. It allows the targeting of specific rules files directly, which lets you test files that aren't yet referenced in the main Prometheus configuration. This ability can be handy in both the development cycle of rule files and for validating automatic changes in said files when using configuration management. We'll be covering these concepts in depth in Chapter 9, Defining Alerting and Recording Rules.

Here's the expected output when checking a rule file:

vagrant@prometheus:~$ promtool check rules /etc/prometheus/first_rules.yml 
Checking /etc/prometheus/first_rules.yml
  SUCCESS: 1 rules found