I'm using a Shure ULXS-14. You can start by checking the published frequency response, but many systems don't show that info. In my experience, those systems have a limited bandwidth and are only usable for guitar. If the system is listed as being flat down to at least 30Hz (preferably 20Hz), then it should work. I used a signal generator and oscilloscope to verify the response on mine.
(Message edited by lmiwa on January 27, 2009)