How do you view the source of a question, to see how the LaTeX was specified as markup?
Another possibility (if you want to copy a large chunk of text with several formulas) is to click on edit (and, of course, not save the edits you make, just copy the part you need).
Viewing source of the webpage also sometime works (View/Source in Internet Explorer, Ctrl+U in Mozilla Firefox). This way has some flaws - you don't see markup, but html. If TeX-contains & (ampersound) it is changed.
The above is explicitly about getting the MathJax source from a post. (This is what was actually asked in the question.) There were a few comments asking how to view the source in comments. For this, see: Seeing the MathJax mark-up for a comment and How to copy mixture of text and latex formulas in a comment? (Although some of the suggestions given in the answers to this question - namely using MathJax menu and displaying source of the website - work for comments, too.)