Check out this video with steps on how to use rain gauges or other cups to measure the water your sprinkler system produces. Knowing this will help you schedule your irrigation schedule.
Below is a chart of how many inches of rain/watering per week required for a healthy lawn. For example if you get an inch of rain in May, you will not need to water your lawn during the week.
If you like measuring precipitation, become a weather reporter for CoCoRaHS, Community Collaborative Rain, Hail and Snow Network.