Option-click on the Wi-Fi Menu Extra. Look at the PHY Mode. You might also be interested in the Transmit Rate and maybe the MCS Index.
Similar information can be found via the Info tab of
/Applications/Utilities/Network Utility.app, or via the Wi-Fi item in System Profiler (or I guess it's now called the "System Report" part of the "System Information" utility).
Note that these things tell you how your Mac has connected to the current AP, not necessarily everything that the current AP is capable of, or what other APs in the same network (if it's a multi-AP network) are capable of.