You can get pretty close by ear, but to do it you need a stereo potentiometer, wired as a stereo rheostat, loading the cartridge. I usually use a 100K device.
Turn down the control until the highs seem to go away and then back off. There should be no change in volume, just the highs, which should sound normal and extended but not bright.
You will be compensating for other factors in your system but without instrumentation this means works pretty good.