How to recognize preview and full post view

Hello,

I style my blog, so I put this ‘related posts’ in index.php:

<?php wp_related_posts(); ?>

But the problem is that related posts are show on the mail page of my blog (where I have only short post introduction and “read more’ link). I would like to do something like this:

if (post_preview())
{
 // nothing
}
else
{
 <?php wp_related_posts(); ?>
}

Is this possible??