Friends of mine in Colorado are going through radon-remediation right now. Bob is a retired electronics test engineer, and after a lot of experimentation with kits and looking into a professional contractor, he purchased one of these:

http://www.amazon.com/Safety-Siren-S.../dp/B011ACOMK0

He said its readings agree with the industry-standard test kits.

They're attempting to do their own remediation first and will be using this detector to gauge improvement. If they can't get it to safe levels, they'll have to put a pump in.