How can I differentiate between third-party and custom modules in the list of modules? If I have administrative access to a Drupal site, I can see the list of the modules enabled for the site. 

How can I differentiate between third-party and custom modules?
You can just do this by looking at sites/all/modules folder - if the sitebuilder was a good one you will see two folders "custom" and "contrib". In modules administration page there is no simple way to differentiate between them.