Hi,
Yes, you could build something to check it but it would be fairly expensive to do right and take some time to do. Measuring flow is expensive...measuring pressure against a fixed orifice and noting the change is probably the quickest and cheapest. But even turning the pump at a precise RPM can be a challenge. If you're off 15 RPM for example, it'll result in a different number.
I'd recommend just using your engine... if you have a data logger you could note your pressure with a particular pill that you typically run at a certain RPM that you always run through. It should be pretty much the same. If you see it running high you could have a nozzle partially blocked or a filter getting dirty (depends on where your sensor is). If you see it showing low consistently, then that is probably pump wear and reclearancing will fix it up.
If you don't see your tuneup getting leaner over time and there's nothing leaking from your pump, I'd just run it. An annual (or every other year) flow test and (if needed) a shaft seal, some bearings and a good cleaning inside from your pump dude (me I hope!
) should be plenty good.
Spud