Update: This answer isn't very helpful. Don't let the accepted status or vote count fool you. See Rahul's answer or MathJax basic tutorial and quick reference.
(Viewing source became more straightforward for those with under 2000 points some time ago, as pointed out in a comment. Just click "edit" or "improve this question". The revision history is still useful for other things, but largely irrelevant here.)
Below is the old answer in tinier typesetting.
Expanding on Arturo's last point, you can view the source of any post on the site, regardless of whether or not you have enough rep or are even logged in. You just need the post number. If the post number is XXXXX, then you can go to
where there is a link "view source".
The post number of a question appears in the URL of the question. To get the post number of an answer, you can use the "link" link in the bottom left of the post, where the post number is at the end of the URL.
For example, here are a question and an answer that as of now have not been edited:
You can do this on meta, too:
You can get to the source more directly by clicking "edit" in the bottom left if logged in, and if a post has been edited then you can click on the time to the right of "edited" in the bottom center of the post.
Without having to go to the revision history, you can look at any MathJax source by right-clicking on the math and then clicking "Show Source". All it leaves out is the dollar signs.