you actually can test a fluid style damper on a balance machine, just not at the typical 500 rpm speed that cranks are usually balanced at.
spinning them up un the 700+ range they will go neutral if they're working right, below that they won't neutral out. once they neutral out you can decrease the rpm to around 600-650 and they'll stay neutral.
that machine shop test that Moparnut mentioned is typical at 500 rpm.

both ATI and Fluidampr brands are expensive. we have some less expensive elastomer (ati) style dampers for those on a limited budget that seem to perform quite well also.