How to determine the modules enabled on a third party website?
There are various you can find this out. But none of them are foolproof and they are just dirty tricks at best.

You can check source(view source) of the website and do the following.


Check the css includes in the head tag and figure out the modules from path.
Check the javascript includes in the head tag and figure out the modules from the path.
Check the classes of html tags from the body tag.
Do the above for multiple standard pages and you should be able to make a list of at-least 50% of the websites based on your knowledge of the modules.


But the easiest of them all is using the http://www.drupalxray.com/ website. Looks like they have automated many rules like the above and search enough paths to prepare a pretty exhaustive list of modules used on a website.