Without some effort you don’t. LoRaWAN is a broadcast system where your node shouts and ALL gws in range hear and pass on the nodes message to the NS. What you worry about is has your node lost its session on the NS. Nodes do not connect to gws……this is not WiFi….they connect to the network. If you look at your device/application data on the TTN console the expanded metadata will show all gws that hear that message. Your gw view in the console will also show if your gw hears the node. If no messages get through then you worry about the operation of the device. If gw view shows no messages it’s a node problem most of the time….even a foreign or unconnected node will show as traffic. If message appears on gw but not in application or device view then it’s usually a session or ‘connection’ issue with the node on the network - check keys etc. If no message on gw view but data appears in device or application view the other gws are carrying the load for you and likely there is a problem with your gw….check it’s connected to NS ok and is powered, has internet connection, etc.
….there are other subtleties and variations to worry about but that covers 90% of overview fault finding ime.