Firefox comes with a built in font inspector, that allows you to see the fonts used on the element you are currently inspecting.

You can modify the default "Abc" text to try the font with any text you would like. At the bottom of the fonts list is a button to display all fonts on the page, allowing an easy way to see exactly what fonts are being used on the page.

Firebug provides an info tips for any font declaration that appears in CSS.