To go to the log-in form on most any Drupal site, simply add /user to the base URL.
So for the site example.com, visit example.com/user and if it is Drupal you should be presented with the sign-in form, plus tabs to send a replacement password by e-mail and possibly to register to create a new account.
If the site does not have clean URLs enabled (which you can see if you visit other pages), you will have to include ?q= in your path: example.com/?q=user.
