According to Advanced Water Systems, the easiest way to measure flow rate is to fill a bucket or other receptacle with a known volume, and time the flow. Once the receptacle is full, use the volume of the receptacle and the amount of time it took to fill it to calculate the gallons per minute.
Using the method outlined by Advanced Water Systems, if it takes five minutes to fill a five-gallon bucket, then the flow rate is obviously five gallons per minute. If it takes 45 seconds to fill the same bucket, then the flow rate is 60 seconds divided by 45 seconds, times five gallons, equaling 6.6 gallons per minute.
On About.com, Bob Formisano writes that measuring the flow rate of a faucet or shower head can be done by placing a container under the fixture and collecting the water for ten seconds; after ten seconds, measure the amount of water in the container and convert the measurement to gallons. Multiply the number of gallons by six to calculate the flow rate in gallons per minute. If it takes ten seconds to fill a container with 0.25 gallons, then the flow rate is 0.25 gallons times six, or 1.5 gallons per minute.