How can I tell if the Pause/Break key is working?

You can test that any key is working with the xev command.

Press Ctrl + Alt + T to open a terminal window, then type xev. Another popup window opens but you can ignore that (it displays mouse activity). Watch the terminal output while pressing the Pause/Break key and see if there is any output. Here's mine:

Note that the window created by xev will need to be the window in focus, otherwise, the utility won't capture the key press.

For how to use this data, or to remap the key, see this similar question.

I've written an in-browser test, so you can test this without having to open the terminal.

Simply press the Pause/Break key and see if it's working.

⇒ Pause/Break key test