This project got some blog love last year, but it slipped past my radar. [jhecker] built a parallel port interfaced device based on a Cypress 2.4ghz transceiver module. The module is pretty complete, so as long as you can wield a soldering iron, you can pull this one off. The module is pretty cheap, so it could be just the thing for building your own signal detector.
http://www.hackaday.com/2008/02/14/diy-2-4ghz-spectrum-analyser/