A fire truck from our local volunteer fire department pulled up our street and stopped in front of a fire hydrant. One firefighter jumped out of the truck with a long poled tool. He removed one cap from the hydrant and then opened the valve at the top.
Water gushed out, bending the nearby shrub and washing mulch down across the sidewalk. Once he determined that the hydrant's water flow was sufficient, he turned it off and closed it up. He got back on the truck and they drove away.
My fire needs used to be served by the city and I don't know that I ever saw they test a hydrant. And now the volunteers are doing it.