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( __( '… Read more about this article <span class="meta-nav">→</span>' ) );
}
else {
the_excerpt();
}