I am taking over this project. The previous developer left absolutely no documentation as to what was done.
I normally log onto 'built with' to find out what technologies were used, but that is not giving me much info.
At this point I am thinking it is on Magento or something, but I can not be 100% sure.
You could use a browser addon such as: Firefox - https://addons.mozilla.org/en-US/firefox/addon/10229/
Chrome - https://chrome.google.com/webstore/detail/appspector/homgcnaoacgigpkkljjjekpignblkeae
If you want to do it manually:
For WordPress - CTRL + F -> Search for "/wp-content/"
For Drupal - CTRL + F -> Search for "/sites/" (I think you may also be able to simply search "Drupal")
For Magento - CTRL + F -> Search for "/skin/frontend/"