Ensure that your well's pump is built up to full pressure by turning off all water outlets and waiting for the pump to cycle off.
Locate a spigot in your home that lies below any type of water pressure tank that may be installed on your water system. Most outdoor spigots will satisfy this requirement.
Turn the spigot on and collect the water in a 30-gallon container so the amount can be measured.
Turn off the water as soon as the well pump cycles on and measure the time it takes for the pressure to build back up, which will be indicated when the well pump cycles off.
Plug your numbers into the following formula: (gallons collected / seconds for pump to cycle off) multiplied by 60 = Gallons Per Minute (GPM). For example, if you collected 15 gallons and it took 75 seconds for your pump to build pressure back up, your equation would look like this: (15/75)X60 = 12 GPM.