Complex sentence: It's hard to know whether or not a radio station is broadcasting solely based off of the information in the REC/Industry Canada databases, however when a radio call sign is missing from those databases it's very likely that it's not transmitting.
Split into simpler sentences: It's hard to know whether or not a radio station is broadcasting solely based off of the information in the REC/Industry Canada databases when information about a call sign is present in them. However, when a radio call sign is missing from those databases it's very likely that it's not transmitting.