One main reason of volcanic eruptions being predicted is the fact that temperatures around the volcano tend to rise as the activity increases. This means that we can use satellite cameras and thermal imaging techniques to detect the heat around a volcano.
Also, another reason would be that hundreds of small earthquakes are caused as magma rises up through the cracks in the Earth's crust. We can use seismometers to detect the earthquakes and in turn detect volcanic eruptions as well.
