How to force excerpts / teasers in the loop

EDIT

OK, there is a very hidden native function, get_extended() that I never knew about and greatly explained by @kaiser in his answer. My answer should only be an extension to the answer by @kaiser

ORIGINAL ANSWER

There is no native function to do this. The best here would be to use a PHP function like strpos to search for the more tag, and then do something according to the result.

You can try something like this

if( strpos( $post->post_content, '<!--more-->' ) ) {
    the_content( __( '&hellip; Read more about this article <span class="meta-nav">&rarr;</span>' ) );
}
else {
    the_excerpt();
}