How to test X11 Forwarding?

Andrew asked:

I am trying to enable X11 forwarding so that I can run Firefox from the remote host. The remote host is a Vagrant VM running Ubuntu 12.04. The local host is Mac OS X 10.10. I think I have everything configured correctly, but I’ve never done this before, and it doesn’t seem to be working.

What is an easy way to test that X11 forwarding is configured correctly?

My answer:

Simply running a graphical program when logged into the remote host is a sufficient test.

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