How to tell if my Wi-Fi is connecting using 802.11n?

Answer from another question:

C:\Users\kuba>netsh wlan show interfaces

There is 1 interface on the system:

    Name                   : Wireless Network Connection 2
    Description            : DW1520 Wireless-N WLAN Half-Mini Card
    State                  : connected
    Network type           : Infrastructure
    Radio type             : 802.11g         <-- the currently negotiated value
    Authentication         : WPA-Personal
    Cipher                 : CCMP
    Connection mode        : Auto Connect
    Channel                : 11
    Receive rate (Mbps)    : 54
    Transmit rate (Mbps)   : 54
    Signal                 : 82%
    Profile                : xxxx

    Hosted network status  : Not started

If you want to see clear solid indicator, simply hover over connected wireless network. You can find Radio Type in tooltip.

In the screenshot, you can see that my Radio Type is 802.11g.

Go to the Network and Sharing Center and select Change Adapter Settings. Right-click your wireless adapter and click "Status". It should say somewhere there whether you're using Wireless-N, Wireless-G, etc.