Hi Emily,

We actually have instruments that can measure the magnetic field at different points on the Sun, so a magnetic map (called a magnetogram) of the whole Sun can be made. When such magnetograms are compared to images of the Sun it is pretty clear that the locations of the sunspots matches locations of higher magnetic field. If you take these images/measurements every few minutes or hours, you can even see how the areas of strong magnetic fields move and change and how that corresponds to changes in the shape and size and position of the sunspots.

Best,
Christina