Garrett Dear Garrett, Think about the ways doctors look at patients. They use different tools and methods to look at the exterior and interior of their patients. Sometimes they even take samples and study them later. Volcanologists still want to see inside, see outside, and take samples. To see inside they look at the distribution of earthquakes.
Earthquakes can indicate where magma is moving.
Just like your chest expands when you take a deep breath a volcanoes expands when more magma moves into it. Volcanologists can measure the expansion. Volcanologists also samples gas that comes out of volcanoes. A change in composition may indicate that new magma is on the way to the surface. Have a look at How to become a Volcanologist , Working on Hawaiian Volcanoes ,and Monitoring .