How can I know which Wi-Fi protocol my MacBook is using? If I have a router that supports 802.11 N, G, and B, how can I know which one of these my MacBook is currently using to connect to the router?

I'd like to see proof that it's connected using the fastest protocol available, rather than hoping everything is working correctly. For example, the N protocol may not be compatible with the MacBook (if it's an early draft), so it would use G instead. Another situation may arrise if the N protocol is disabled on the router via its settings, which would also cause G to probably be used. Thus, rather than assuming that it's using the fastest protocol, I want actual proof that shows which protocol is being used.
Try to Option+click on the wifi icon on the menu bar. You'll see a lot more information about your current wifi connection.

It looks something lijke this: