How do we know where volcanoes will erupt?
One of the ways is to look for evidence that there is a magma on the move. We should see swarms of small earthquakes and some pushing up of the earths surface as the magma moves up.
Take a look at the Three Sisters area in Calf. --it went up 15cm in 4 years --that is huge. Check out the radar data in this article:
Monitoring Volcanoes from Space