Find patches of different types of soil that are on slight slopes or hillsides. You could also look for patches of the same soil type with different amounts of plant cover, for example, bare soil and a grassy area.
For each test site make a sampling container. Cut the tops off plastic bottles such as soda bottles or milk jugs. Use the same size bottle for all sites. Bury each container so the lip is even with or slightly below the soil surface.
Weigh the soil that collects in the containers after each rain. Dry the soil in an oven before you weigh it; you don't want to weigh the water, just the soil. Record the differences to find out which test site had the most erosion.