I see other posters writing beautiful integrals and quotients within quotients and matrices and sigma summations and and and... is there a guide for that?
(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.
The equations are typeset using $\LaTeX$; there is a link that says "MathJax Help" when you are editing, which links to Kuhn, Scott, and Andreev's Introduction to LaTeX. Most of the formatting is supported, though every once in a while there are a few things you need to do to get MathJax to work right (like extra
\ to act as escape characters, etc).
(You can learn some tricks by looking at how some of those people formatted their formulas; you probably can't do it with other people's posts yet because of your reputation, but you can do it to any posts you make if someone edits and formats them).
Seeing as this is the featured FAQ post about entering mathematical notation on this site, I feel it's useful to consolidate here the links to some more useful $\LaTeX$ resources that have been pointed out in another question.
This is a list of the $\LaTeX$ commands currently supported by MathJax.
You can use the CodeCogs $\LaTeX$ equation editor to form $\TeX$ expressions you can then paste into here.
Also useful is Detexify: you draw the symbol and it performs OCR to tell you the name of the symbol.
neutriNote (last updated: Dec. 20, 2017) - Android APP for note taking. Includes speech-to-text and OCR of $\LaTeX$. Free at the Google Play Store. https://play.google.com/store/apps/details?id=com.appmindlab.nano
Feel free to add any others that may be useful to users new to $\LaTeX$.
Also very helpful is Carol Fisher's Alphabetical List of TEX Commands available in MathJax, which gives examples of all MathJax commands, and has a little MathJax sandbox for experiments.
I think someone had already mentioned WP:MATH somewhere. You can just type
WP:MATH in the firefox's English wikipedia search.