All you need to do really is measure the amount of time it takes the pump to fill a certain container (the larger to container the better). Then measure the size and container, convert it to gallons, and divide it by the number of minutes it took to fill that container.
Here is an example:
Let's say your container is barrel (cylindrical in shape) 18" diameter and 40" high, and it takes 20 minutes to fill.
Radius of barrel r = 18"/2 = 9"
Height of barrel h = 40"
Volume of container = π * r^2 * h = π * 9^2 * 40 = 3.1416 * 81 * 40 = 10,178.78 cubic inches.
Since there are 231 cubic inches in one gallon (US), you divide the cubic inches by 231 to get the volume in gallons.
Therefore volume of barrel = 10,178.78 / 231 = 44.1 gallons
Therefore the pumps flow rate in gallons per minutes = 44.1 gal / 20 min. = 2.20 gallons per minutes.
Hope this helps.