802.11c or a 802.11ac, how do I find out which I have?

There are 2 ways to find out which model of Time Capsule that you have using AirPort Utility

Open AirPort Utility

Click on the Time Capsule to select it, and another smaller window will appear

Hover your mouse over the name of the Time Capsule and another window will open with info about the Time Capsule

If you see AirPort Time Capsule 802.11ac.......you have the 802.11ac version.

If you see AirPort Time Capsule 802.11n.......you have an older version of the product.

Alternately.......

Open AirPort Utility

Hold down the option key on your Mac while you double-click on the picture of the Time Capsule

Information about the Time Capsule model, serial number, ID's, firmware and other boring stuff will appear

Pretty much impossible to have an 802.11c device. The commercially released versions of the wireless communication standard are: 802.11 a, b, g, n, ac and the newest one is ax.

802.11c was never implemented in commercial networking products.

If you have semi recent time Capsule its an 802.11ac type of wireless networking device.