Atomic Red Team™ by Red Canary is a library of simple tests that every security team can execute to test their defenses. You can use it is a service to run MITRE ATT&CK techniques against your tenant (live or staging organization) to see what you have coverage against, and to uncover any gaps. You can learn more about the Atomic Red Team on their official website.
Once you have found the gaps in your detection & responses rule coverage, you can write D&R rules to address these gaps.