There are 3 main ways in which a volcano is monitored to see if it is nearing an eruption.
1. Earthquakes (or as we call it, ‘seismicity’). Before an eruption magma has to move through the volcano. While it is moving it breaks rocks apart to make a path for itself. Where the rocks break, this causes small earthquakes that we can record with seismometers. If we see an increase in the number of earthquakes, or if they start getting closer to the surface, this may indicate an eruption is going to happen.
2. Deformation. Because magma is moving and building up beneath the volcano, the rocks around it are also pushed around. Imagine burying a balloon in a sandpit and then blowing it up with a pipe. As the balloon inflates the sand above it will rise up. This is similar to when a magma chamber inflates from new magma. So we can measure this deformation using GPS or satellites and if we see an increase in the movement this may indicate an eruption.
3. Gases. As magma moves towards the surface the gases that are dissolved in it start to come out. These can make their way to the surface where they can be measured with equipment based on the ground or in satellites. By recording the temperature and type of gases coming out we can get an idea of what the magma might be doing.
These are the 3 main ways, but I must say that it isn’t always this straight forward. Sometimes we can see signs that a volcano is going to erupt and it doesn’t erupt. Or we don’t see any signs and it does erupt. This is why it is very important for people to study volcanoes so we can better understand why this happens.