The SSID is whatever name that you gave your 5GHz band in the AirPort Utility. Just open your AirPort Utility and select your Time Capsule. Click on the Wireless tab and you should see something like this...
If your Time Capsule is dual-band, the SSID or Wireless Network Name will be "Extreme ac" as illustrated above. This will be your SSID for your 2.4GHz band. To determine what the SSID is of your 5GHz band, click on Wireless Options...
If you decided to give your 5GHz band a different name (SSID) to distinguish it from your 2.4GHz band, note the blue check mark next to the 5GHz network name. As illustrated above, the SSID it will be displayed in the 5GHz network name field as "Extreme ac (5GHz)" or whatever name you gave your 5GHz band.
If you did NOT give your 5GHz band a different name, your SSID will be the same name for both bands, 2.4GHz and 5GHz.
Hope this helps.