When you want to see a preview of a font in Mountain Lion from the fonts panel, just click on the little dot under the title and pull downwards. This will reveal the font preview section of the panel. From here you can preview the font in whatever size or style you want. Also, you can make adjustments to the appearance of the font.
If you're not familiar with the Fonts panel it can be found in TextEdit by pressing Command+T or by right clicking on text or a textbox, going to Fonts and clicking Show Fonts (thanks OSXDaily for the 2nd method).
You can also manage fonts from the Font Book application.