The real problem is the variety of frequencies and spreading factors only one of which can be monitored at a time with a node-class radio.
The encryption isn’t much of an issue for “does it work” testing of one’s own node because the device address is cleartext not encrypted, so unless there’s someone hiding in the bushes trying to be excessively funny by also sending packets from that address, you can tell when you have a transmission from your node. Also you can generally extract your own keys from the node or server.
Of course having mis-entered ABP keys could be a reason why the node isn’t manage to get through to the TTN servers…
An RTL-SDR also works at the level of noticing that a nearby transmission happened (likely across most if not all uplink frequencies, and all spreading factors) though most don’t attempt to use that to actually demodulate LoRa, but just see something with the characteristic chirp appearance.
This (with the result of getting the raw packet and status logs) is indeed the best solution, really the only cost effective one when considering the economics of time spent frustrated wondering what is going on