You probably know whether or not you're near-sighted, but some people get so used to seeing things a certain way that they ignore a vision problem, squint a lot, and end up with unnecessary eye strain at the computer.
The double-image above cuts straight to the point: If you see Albert Einstein while sitting a normal distance from your computer, you're seeing things as you should. If you see Marilyn Monroe, you should probably be wearing glasses or contacts.
I've got glasses, so I can easily A/B test by taking my glasses off (I see Monroe) and putting them back on (I see Einstein). If you aren't near-sighted and you want to see the image how near-sighted folks do, you can squint or just walk away from your computer until you see Monroe.
We're no optometrists, but it's easy to ignore a vision problem that's snuck up a little bit at a time and doesn't hurt to check.
PS If anyone knows where this image originated, let me know and I'll update the post.