You can check the source code or select some text, right-click and pick "inspect element". You'll probably find a list of fonts and you won't be able to tell which one is actually used.
Chrome added a feature that tells you the name of the font that's used. This feature is available in Chrome 31+, so you can try it if you've switched to the Dev Channel or you use Chrome Canary. Right-click the text, pick "inspect element", switch to the "computed" tab, scroll down and you'll see "a summary of the typeface(s) used for that element". Paul Irish says that it "works great with Google Webfonts, Typekit, local fonts, @font-face typefaces, unicode glyphs, and all other interesting font sources."
As you can see, it's Helvetica, not Arial. No need to use WhatTheFont or WhatFontIs.
For some reason, this didn't work well for web fonts.
{ via Kristian Serrano. }
