Open the tap full blast, place the container underneath the flow and time the amount of time it takes, in seconds, to fill the container.
Example: 5 gallon bucket takes 15 seconds to fill
5 gallons --- 15 seconds = 5 / 15 * 60 = 20 gallons per minute average flow rate

Alternately you can take a certain amount of time and then measure the amount of water in the container.
Example: 5-gallon container placed under the water flow (full blast) for 10 seconds, remove the container and then measure the amount of water in the bucket. If you are using a measuring cup you need to convert this to gallons. One cup = 0.0625 US Gallons. If in 10 seconds you have 48.5 cups of water you have 48.5 * 0.00625 = 3.03 US gallons of water in your container. Enter 3.03 under container size and 10 seconds in the time it takes to fill the container.
3.03 US gallons---10 seconds = 3.03 / 10 * 60 = 18.18 US Gallons per minute average flow rate

To get a more accurate result all other water sources in the house should be turned off.