On a Mac computer, you can retrieve extended wireless connectivity data and details from anywhere within Mac OS X by holding down the Option key and then clicking on the WiFi menu.1 (See attached image).
Some of the more useful data:
RSSI (Received Signal Strength Indicator) is a common name for the signal strength in a wireless network environment. It is a measure of the power level that a RF client device is receiving from an access point. The closer the figure is to zero, the better. For example, an RSSI of -65 is better than -85.2
Noise - The noise level indicates the amount of background noise in your environment. The closer the value to 0, the greater the noise level. For example, -96dBm is a lower noise level than -20dBm.3