There's no way to test hyperlinks in an email you're composing without carefully selecting the whole URL, copying, switching to a browser, pasting, hitting return -- quite a bother if you're writing a newsletter, for example, with lots of links.
But there's a simple solution: Save your email as a draft with Command-S, then look at the email in the drafts folder of the Message Viewer window. In that view, all links are highlighted and clickable.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20100322171722145