Recently, I discovered (or subconsciously recalled) how to preview retina favicon's without a rMBP. You can't see the favicon in Xcode's iOS simulator (iPhone and iPad Safari tabs don't display it), so options are limited to a trip to the Apple store, or asking a friend who uses a rMBP.
Or so I thought.
Thankfully, Chrome for iOS displays the retina version of the favicon in it's synced tabs screen. What's especially cool is that it will show you if you didn't follow the Daring Fireball directions correctly. The result? A fuzzy retina favicon (ZenPen).
Bad: take a sufficiently large image and drag that image into BOTH the 16x16 and 32x32 wells in Icon Slate.
Good: create two separate 16x16 and a 32x32 versions of the source image and place them into the corresponding wells.
If you end up with a fuzzy retina favicon, now you know why.