How do I find which TC are AC capable?

The only TC that is "ac" capable is the current "tower" shaped version of the product.

As far as the speed capabilities of your current TCs, we would need a model number from the bottom of the device to be able to tell you more about each product. The model number starts with an "A" followed by four numbers.

No matter whether you have an "ac" versions of the TCs or not, you might want to remember that whenever you "extend" a network using a wireless connection between the AirPorts, you will lose about half of the potential speed capability on the network.

Hopefully, you have the TCs connected back to your "main" router using wired Ethernet connections.

All "tower" AirPorts and Time Capsules are ac compatible.

I think one of my Time Capsule's a first tower release before they were touting AC capable. I'm not positive however when I bought it years ago I don't think Apple had any AC hardware. That's why I'm trying to determine which one is. When AC capable Extreme's and TC's towers were released they also boasted improved antenna array. I think there's a little more to it, which is why I'm trying to decide if replacement could improve coverage also.

There has only been one model AC wireless.. the tower model.. anyway it is easy to find out.

In airport utility hold down the option key and double click the icon. This will give you the full summary page. (click the icon gives the half summary).

Thus and so.. Model says explicitly AC ..

AirPort Time Capsule 802.11ac

I think one of my Time Capsule's a first tower release before they were touting AC capable

Unfortunately, you seem to be confused.....and the answer is very simple.

If your TC looks like a tower.....then it is "ac" capable. If your TC is "flat or square", like a pizza box, then it is not "ac" capable.

But, there were four versions of the "flat or square" TC, and the wireless specifications for each one are different. If you want to know more about the versions that you have, then we need the model number(s) of the TC(s) that you have.