<?php
// Get $post if you're inside a function
global $post;

if ( has_excerpt( $post->ID ) ) {
    // This post has excerpt
} else {
    // This post has no excerpt
}
?>

Other Use

When you need to hide the auto displayed excerpt and only display your post’s excerpts.

<?php if ( ! has_excerpt() ) {
      echo '';
} else { 
      the_excerpt();
}

Replace auto excerpt for a text or code.

<?php if ( ! has_excerpt() ) {?>
    <!-- you text or code -->
<?php } ?>