How can I measure how much water my sump pump pumps?

Wait until the sump is full and ready to turn on. Put a 5 gallon bucket under the pump output line, turn it on and measure the time it takes to fill the bucket. Divide 60(seconds) by the number of seconds it took and then multiply the answer X 5(gallons). E.G. if it takes 10 seconds to fill the bucket, 60/10=6. 6X5=30. 30=GPM. Most pumps are rated in gallons per hour which would be 30X60 or 1800 GPH