Take a hose and place it into the downspout. Have a helper turn on the water and check for downspout flow. If it's clogged do the following to clear it out:
- Pack the downspout opening around the hose with a rag sealing the hose tight.
- Turn on the water pressure as hard as it will go, looking for the clog to clear and water to flow freely from the bottom of the downspout.
- If the clog persists you'll have to clear it. Remove the hose and manually clear the clog using a plumbing snake. Feed the snake from the top of the downspout until it hits the obstruction which is usually at the point of a curve.
- Once you think you have it cleared, test for flow with a hose.