Line of sight isn’t always required. It all depends on things like distance, antennas use, what kind of obstacles are between transmitter and receiver, RF noise floor in the area…
If you aren’t near the gateway sending a return packet chances are small you’ll be able to see it using SDR. Lora modulation is made to be detectable below the noise floor, in a visual inspection of the spectrum it will be hard to detect.
Take into account you can’t always predict which gateway will be chosen to send downlinks in an area where multiple gateways receive your nodes transmissions. Usually the closest gateway will have the best signal conditions and will be chosen, but airtime and gateway use are factors as well.
Using a generic SDR dongle and Windows software I haven’t been able to scan the entire band used by EU868 with sufficient speed to be able to capture all transmissions of my test node (at least I wasn’t able to recognize them).