As per Carl, you can roughly figure energy into the pool at flow rate * temp differential. This doesn't tell you how much energy is going in, but it gives you a way to compare whether a given flow rate is better than another.

From my experience and from my research, it seems pretty clear that you want as much flow as you can possibly put through the panels without stressing them. Most panels are rated to handle flows up to about 8 to 10 gpm (for 4 foot wide panels) - BUT, the efficiency curves drop off above about 5 gpm. So, most solar installers figure 5 gpm per panel (in a parallel configuration).

So, if you have only a few panels, you can run a 2 speed pump at low speed (if there isn't too much head loss) or you can use a diverter with a 3 way valve to adjust the amount of water going through the panels. If you can touch the panels, you can just keep increasing the water flow until the panels are cool to the touch.

If you have enough panels, you can pretty much run wide open. I run my 14 panels on a multi-speed 1.5hp pump at close to max, with a calculated flow rate of about 80 gpm (and go to really low speed when the panels are switched off). WooHoo! I'm at 85 degrees in NY in May, my neighbors are only now opening their pools to water in the mid-60s. The water coming out of the panels is noticeably warmer (on a sunny day) than the pool, but maybe only 1 or 2 degrees warmer than the pool.