Installing a flowmeter in a residential pool is a little unrealistic because of space limitations. And you cant really just 'take one out to use on the next house' because they are calibrated for a certain pipe size diameter, and on top of that, you need to drill a hole in the pipe and secure it with hose clamps, meaning you would have to plug the hole every time.
Per the 2009 Pool and Spa Operator Handbook (CPO book): "When water changes flow direction, a certain amount of turbulence is created. Because of this, there is a requirement of ten times the pipe diameter of equivalent straight pipe before the flow meter and four or five times the pipe diameter of equivalent straight pipe after the flow meter."
Most residential pipe is 2 iinch, so that means you would need 20 inches before the meter and 10 inches after it. Not many people have 30 inches of straight pipe on their equipment pad...unless you feel like doing some digging to the underground piping.
The next best thing you can do is to see what pump it is and look at the manufactures web site and find the pump head graphs and match it up to your current TDH, which will give you a fairly accurate reading on what your flow is.