There are free tools, such as NetStumbler, and more capable ones, such as AirMagnet Survey, Visiwave, Fluke Networks InterpretAir and Ekahau Site Survey that you can use to test the signal strength. To boost any router, you should get a wireless repeater. All routers in essence are the same. This is what I would do,
- Purchase or use your existing wireless router and replace the “firmware” with DD-WRT (open source firewall (this can be your main wireless router)
- Purchase or use another existing wireless router and replace the “firmware” with DD-WRT (this can be your wireless repeater)
Instructions to download the DD-WRT for your router (locate your router to verify support)
Instructions to install DD-WRT
Instructions to add the repeater to your wireless router (Step 1)
There are alternatives to changing firmware. You can add a higher-gain (stronger) antenna or make the current antenna dipoles more direction toward the client devices. Consider this method for boosting wireless signals. Additional videos on this topic may be found here. I personally like the idea of using a metal mesh strainer as a RF signal concentrator.
It would be better to upgrade your antenna, tweak the gain and/or buy a better (enterprise/small business AP/router) than tweaking the Firmware. Open-source =/= guaranteed to work, and changing to a 3rd=party FW will void most warranties.