Test Retina Images Without a Retina Screen

This comes directly from a tip Mike Zarandona posted on Coderwall, which I'm posting here to keep track of it:

I was sick of borrowing a friend's retina laptop to test retina images. After a little poking around, I discovered a few options baked right into the most popular browsers.

In Google Chrome:

• Browse to chrome://flags
• Search for and enable Force high DPI mode

In FireFox:

• Browse to about:config
• Search for layout.css.devPixelsPerPx