From the Tools menu, select Co-observability. This will run an automata-based algorithm to verify that the synchronous product of your project supervisor DES is co-observable with respect to the synchronous product of your plant DES, and your decentralized control system. Note that your supervisor should be controllable for your plant for this to be useful.
To aid in specifying the system's decentralized supervisors, DESpot has three editors: Edit Observable\Controllable Events, Add Decentralized Supervisor, and Remove Decentralized Supervisor. They are each described in the links below.