Or you may go to Settings>Network and Sharing Center>Change adapter options. Once you’re there, navigate down to Network -> Wi-Fi or Network -> Airport depending on how old your MacBook and version of OS X might be, and you’ll see the current connection over on the right. cpl->right click the Wi-Fi/Wireless network connection->Status. You can open up the System Information application from Spotlight search, or through Applications -> Utilities.
This rate will change as you move around your house, so if you’re far away from the router, the rate will change, and if you’re closer, it will get higher. Once you’re there, you can see the current connection speed by looking at the Link Speed, which will show the actual data rate that you’re using.
You can use the Network Utility by using Cmd+Space to pull up the Spotlight search box and typing it in, or you can navigate through your Applications -> Utilities folder to find it. In the Terminal window, type ping followed by the routers IP address, and then press the Enter key on your keyboard. As pointed out by anon in the comments, you can simply hold down the Option key while clicking on the menu bar icon for your Airport, and it will show you the extended information in the popup display.
