Enable logging for the discovery service. Looked at Logging Level under Controller Settings, and do not see anything that seems to relate to discovery in the list of Services.
Discovery is handled by the apic-em-inventory-manager-service.
But unable to see the service you suggest, that seems to relate to inventory or discovery.
I assume you're trying to debug a specific problem with discovery. See this post for enabling debug and getting the logs.
There is no logging for inventory or discovery in 1.2. This is a known issue that might be addressed in 1.3.