How to tell if Macbook is using 802.11ac
When you're connected to the Wi-Fi network, hold down option and click the Wi-Fi icon in the OS X menubar. You'll see additional details about your connection, including one item labelled PHY Mode:. That will show you whether you're connected via 802.11ac, n, g, etc.
There's also a Channel: entry that tells you the radio frequency and channel.
Hold Option key (alt) and press on your WiFi icon.
That will instantly tell you if you connected with 2.4 or 5 Gig in the expanded menu.
