Even without DOs there have to be ways to check up on the COs to make sure they toe the line. Of course the BOEs could always rat them out, if they were brave enough to contact Bethel directly.
Lately in my area there are several substitute COs. These are the overzealous guys who graduated from BSCC or SKE and make substitute visits to congregations on their own dime. They often tag along with the paid CO for "training". They could also rat out a wayward CO.
Then there are the representatives that are sent from Bethel to give talks at Circuit Assemblies. Of course they have to submit a written report about their spy trip, in which they can also report any missteps the CO might have taken, and of course would be quick to point out any failure to follow the countless rules to the letter.