If I see a channel is offline for some period of time (couple hours or so). Then I have a script I run that checks the gossip my node as received to see if other nodes also see it down. If a high percent (measured in channel capacities) of the down node's peers also say it's down then I figure it's down and try and contact the node operator.
Same as looking at the node's channels on Amboss except I not hitting a 3rd party site.