something like is_childtheme()

in the functions.php of a parent theme, I would like to check for wheter the theme is being run directly, or it is run via a child theme.

Is there a conventional way of doing this?
I cant find the is_childtheme() function I would have liked to have found.. so whats the best way? Check for theme name and compare it to parent theme’s maybe? how can this be done?

As One Trick Pony said, there’s a dedicated function (since WP 3.0):

is_child_theme()