If you have a wireless notebook PC with NetStumbler, you can get a lot of information about nearby wireless networks and relative signal strength. That allows you to experiment with channel, position, orientation etc and watch the results in real time.

Brontide is likely right that the other network couldn't be overpowering yours if you are standing right next to the router. It's an inverse-square law - if you're 2 feet away and the other router is 50 feet away, the relative signal strength is 25^2 = 625:1. The other router would have to be beaming an illegally-boosted signal directly at your position to overcome that.