Which Devices is my DataSource monitoring?

DataSources are applied to devices using the AppliesTo field, which typically specifies a type or class of devices on which a given module should attempt to gather data.

To see which devices a datasource is actually collecting data from, find the datasource definition from the Settings | LogicModules | Datasources section and select "Show Associated devices" from the More dropdown in the upper right hand corner of the datasource:

This pops up a form showing all devices that have this datasource associated with them, and the instances on those devices (Or "No instance", if the datasource is associated with the device, but no instances has been detected by Active Discovery or defined manually.)