They make little fan speed controllers that basically have a dial on them to turn down the voltage (speed). Grab one, unplug the fan from its header, plug the fan speed block into the header, then plug fan into fan speed controller output.
It won't tell you the rpm, but you can guess or set it what you want. They are usually turn knobs and analog. Not fancy but it works.
"Too much is almost enough. Anything in life worth doing is worth overdoing. Moderation is for cowards."