Do you have a check valve on the return side of the solar panel? If you do, you can disconnect your line from the pressure side of the check valve (this is where quick release connections are nice), turn your system on and allow the solar heated water to drain into a bucket with gallon markings and time how long it takes to fill. Adjust you 3-way valve to where you want it and mark the valve for future reference. When your done you can pour the water back in the pool and reconnect.

That being said, I am no pool or hydraulics expert, and the above test does not acount for the pressure in the system when it is connected. I would assume that the gpm would be higher when it is disconnected and lower when it is connected because of back pressure at the return side? I am sure one of the mods can answer this with greater clarity and expertise.