How to check a filter are applied

You can use has_filter() to check for registered filers.

Example:

add_filter( 'the_content', function( $content ) 
{
    if ( has_filter( 'the_content', 'pre_content_filter' ) )
        return 'pre_content_filter() is active<br>' . $content;

    return $content;
}, -1 );