How do I know if my changes have been saved?

In the upper right corner of the window is a small indicator showing whether your changes have been saved. If a small checkmark is shown, all of your changes have been saved:

If you have changes that have not yet been saved, you'll see a small progress spinner:

And if there was a problem saving your changes, you'll see a warning icon, and after a short while also a warning popup that you'll definitely notice: