It’s actually pretty easy, but worth noting:
ssh ec2-user@ec2machine sudo yum install xauth xeyes exit ssh -X ec2-user@ec2machine xeyes
In short, the magic’s in installing Xauth and then re-logging in with
ssh -X flag.
And I admit, xeyes isn’t strictly necessary, but I find it’s a nice, lightweight confirmation that both display and mouse are working as expected.