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