babark: First they'd write out the test cases, possibly something along the lines of:
1) When the game is launched,
the publishers branding video should play 2) If clicking X, Y and Z key
during the branding video, the video cannot be skipped 3) When the intro video finishes, the menu should show
4) Moving the mouse over the menu items should highlight the specific menu item
5) Moving the mouse away from the menu item should unhighlight it
etc etc and so on.
Things that can be automatically scripted to be checked are automatically scripted to be checked, and things that cannot be automatically scripted to be checked, are then checked manually.
Bugs and unexpected behaviours are then logged and prioritised to be addressed by the developers.