How do I find out how much space left on Time Capsule?

You should find both the total space & available space within System Preferences. (ref: System Preferences > Time Machine)

I followed this thread to System Preferences and to Time Machine, but the space available does not show. I have another external drive that does show its available space but the Time Capsule does not.
Any suggestions? I know it is working because my hard drive recently crashed and I was able to restore everything through Time Capsule.

Click on the clock shaped Time Machine icon at the top of your screen and select Open Time Machine Preferences.

Now go back to the clock shaped icon and select Back Up Now. You should see the available space during the backup process.

Thanks, that worked but why doesn't it show up in System Preferences as you suggested to Rick Au?

I didn't suggest anything to Rick Au. Tesserax did.

It works the same way for me in System Preferences. I can't explain why it would not for you.