Hacked! Is an extremely powerful Drupal module available in both Drupal 7 and 8. Hacked! allows you to check Drupal's modules and core against Drupal.org stored versions to make sure they have not been tampered with.
This module is a great way to ensure that the modules are safe and have not been modified.
It's worth noting that Hacked! will not check any code that does not exist on Drupal.org. However, as Aimee Maree points out, Hacked! can be very useful for finding when an errant developer has modified a module's code.