Most OS X apps have a small "Fonts" window that pops up if you want to change your font's options, but by default, it doesn't show you what each font actually looks like. Here's how to show a preview of every font with one click.
This may be one of the most annoyingly hidden Mac features out there, but when you open up the font window, you should see a small dot under the title bar. Click it, drag it down, and you'll get a full preview of whatever font you've chosen, complete with the size, typeface, and other settings you've chosen. Why this isn't on by default we'll never know, but apparently it's been hidden there for years.
Just realised this secret of the Fonts panel by accident. Wonder if anybody knew. | Reddit via Cult of Mac