Most likely seismograph logs, I guess? Not sure gas emissions are something we can expect to provide a more ahead of time alert. That is my guess is that by the time some out of charts in gas emissions are showing, it's too late to make a whole city population move? I'm nothing like a volcanologist though, this is just very wild guess.

On the other hand, it's a bit wild to build a whole city next to volcanos that are definitely going to wake up in less than a few centuries, to begin with.