HMaster UI + a JMX field should warn if Major Compaction hasn't occurred and completed successfully after the scheduling interval hbase.hregion.majorcompaction (default 7 days).
Recent changes in HBase
HBASE-6028 mean major compactions could be cancelled and result in read amplification over time. HMaster UI should monitor time since last major compaction completed successfully isn't greater than hbase.hregion.majorcompaction (plus some fudge factor).
If hbase.hregion.majorcompaction is zero then disable the check, as external tooling will trigger major compactions off peak and should instead monitor against JMX timestamp of last major compaction as per HBASE-20945.