Mac: Want to know how good your Wi-Fi network really is? If you have Mac OS X 10.7 or above (Lion or Mountain Lion), a handy but hidden built-in utility can monitor your network's performance and show your network's signal strength.
OS X Daily has posted the instructions for running the Wi-Fi Diagnostics tool. First, you have to find it. You can either go to the folder /System/Library/CoreServices or hold down the option key while clicking on the Wi-Fi icon in your menu bar and select "Open Wi-Fi Diagnostics..."
Open the tool (if you browsed for it by folder, you can copy the app to your Utilities folder or Dock first). Ignore the first screen and hit Command+N to open the "Network Utilities" window.
Hit "Wi-Fi Scan" and you'll be presented with a list of all the networks near you — even ones that are hidden, as well as their channel, security, and signal and noise levels.
If you have competing networks on the same channel, you could possibly improve your reception by changing your channel on your router to one that's not used.
Technorati also explains how to tell if you have a good Wi-Fi signal:
The "Signal" number specifies the signal strength between your Mac and the Wi-Fi access point or router. The higher this number is, the better. But note that these are negative numbers so a Signal of -60 is higher (and stronger) compared to a Signal of -80. The Noise number represents the amount of wireless noise that can interfere with the Signal. In this situation, we want lower numbers. So again, because we have negative Noise numbers, a Noise level of -94 is better than one of -90.
Finally, we can take the Signal and Noise numbers to come up with a Signal-to-Noise ratio (SNR) for our wireless connection. So for example, If I have a Signal level of -60 and a Noise level of -91, the difference between these two numbers is 31. The higher the SNR is, the better the Wi-Fi performance will be. Typically a SNR of 25 or higher will give you great Wi-Fi performance.
If you run the diagnostics from the Wi-Fi Diagnostics screen (or "Monitor Performance" if you're using Lion), you can see a chart of your live signal strength and noise meter. Basically, the wider the gap between the yellow and green lines, the better.
This could help you decide if you need to do some adjustments to boost your Wi-Fi, such as moving your router to a different location or making your own antenna booster.