When I worked in automotive testing we had flow meters that basically consisted of a pipe with a little turbine inside that was attached to a rev counter of sorts. You'd have to hack up a radiator hose to install it, and they aren't cheap, so probably not what you're looking for.

Edit: On second thought, you could get a pretty good estimate of coolant flow by measuring the temperature of the fluid going into and coming out of the radiator. If you come up with a situation where the energy input to the system (quantity of fuel burned) is known and fairly constant for at least a few minutes, you could calculate the corresponding mass flow through the radiator. If you can figure out your fuel rate at highway speed, you could take measurements that way.

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Whoever said there's nothing more expensive than a cheap Mercedes never had a cheap Jaguar.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 401,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 26,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.
99 Mazda Miata 183,xxx miles.