I haven't done any keg hopping since I've started using kegs as primary, so I can't really comment on that.
As far as telling when the keg is full, I cold crash before jumping to my serving keg. Since the beer going in is ice cold, I can just watch the condensation line on the outside of the keg and feather the gas once it gets near the top. You can also tell by sound if you use your spunding valve as a vent when filling the keg. Or you can just wait until it starts spraying out of the spunding valve. Been there, done that, mopped it up.
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