The most obvious indicator is one that is easy to overlook when you are in the same building every day. Familiarity with an environment makes it harder to notice gradual deterioration, and what would have been unacceptable when a contract first started can quietly become the new baseline.
It is worth carrying out a deliberate walk around your premises with fresh eyes. Check whether desks and hard surfaces are being wiped down regularly, whether bins are emptied on schedule, whether kitchen areas are properly maintained and whether washrooms are being cleaned to an appropriate standard throughout the day rather than just once. If you are finding that tasks are being missed, inconsistently completed or only done properly following a complaint, that is a meaningful pattern rather than an isolated incident.
A professional commercial cleaning contractor should be maintaining consistent standards as a matter of routine, not as a response to prompting.