The easiest way of monitoring a volcano is just by looking at it!
Scientists regularly visit the volcanoes to make all kinds of scientific observations using sophisticated instruments. However a pair of eyes is often a very useful tool. By making visual observations volcanologists can note things such as the amount of steam coming out of the ground at a particular spot, any rockfalls, changes in areas of warm ground, and the development of new fumaroles and hot springs, all of which may indicate changes in the state of the volcano.
Images from the cameras are made possible with the support of the following organisations:
more detail here