To get started, press the Option key and then click the Airport/Wi-Fi icon on the top of Mac OS X. This displays some more details on your current Wi-Fi connection while also making the Wireless Diagnostics shortcut available.
When you open Wireless Diagnostics, it will begin running tests to detect any issues, which you could utilize if there are issues. Otherwise you can choose Window > Utilities to access the other wireless tools. You can see the current connection and environment details, perform frame captures, configure logging, scan for network details, and view performance information, including noise and SNR values.
For more info, Apple provides a great tour and tutorial on Wireless Diagnostics.
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