Figuring out what CMS a website is using can give you clues on how to achieve similar results with your own site. While the following method is by no means guaranteed to provide you with the information you are looking for, you'll be surprised how often it does.
The approach is simple. Just add one of the following words to the end of the root of the site's domain:
For example, www.yourDomain.com/wp-admin will display the WordPress login screen. Naturally there are ways to change this and many may well have for security reasons.
It is also possible for someone to have more than one CMS on a site. In this case, each CMS might be in a different folder. So if you notice that all the sites pages appear to be under www.yourDomain.com/blog, you might want to try adding one of the above words to that. For example www.yourDomain.com/blog/wp-admin.
Add a comment if you know of any alternative words.
If that doesn't work, try looking through the HTML source code for clues.
Bonus: Plugins and Themes/Templates
Speaking of which, uncovering some of the the plugins and theme that a site is using can often be discovered by looking through the HTML source code but this is as old of a trick as HTML itself.