"Their plumber?" Sounds like you are getting hosed.
First, find out what the bank requirements are. What do you have to prove? Gallons per day? Gallons per minute for how many hours? Find the minimum flow that will satisfy them. You don't really need to know what the well will do if it has more capacity than required.
When you get the requirements, here is a do-it-yourself flow test.
1. Connect a garden hose to your system at some convenient place.
2. Turn off all water uses in the house.
3. Open the faucet to the hose until the pump comes on.
4. Adjust the faucet until the hose flow balances pump capacity, which you will know when the pump manages to hold a constant pressure between on and off pressure switch settings.
5. Measure the gallons per minute (water meter (best); or a bucket and a watch with a sweep second hand) and let it pump until you meet the requirements. You may have to shut it off from time to time if the pump capacity exceeds the well capacity.
6. Keep track of the time that it is pumping. You could connect an old 115 Volt electric clock to the circuit to record the time.
Another way is to let the pump cycle on and off, and determine the amount of water per minute or hour of actual pump running time. Then put a clock on the pump and run enough water to show that you meet their requirements.
A water meter can be had for about $50 from USA BlueBook (Phone 800-548-1234; Stock number 62276, $31 + shipping and they accept credit cards).