If you want seamless roaming between two APs, then install two *identical* APs, and configure them identically with WDS.

Problem solved.

If you want to know what other WiFi signals are around and interfering, then install Tomato on the router and just click the built-in "scan" button to see what's going on.

But if you have an Android based smartphone/tablet with WiFi, then use the "Wifi Analyzer" app instead -- very handy that.

Cheers