Display code if title of the custom post matches title of other post

You don’t mention exactly if you are referring to the default Custom Fields. If so, the solution bellow doesn’t apply.

But, much better than the default, is to create a Custom Meta Box where you present your Custom Field(s).

The Codex has an example in Function_Reference/add_meta_box.
These can also be useful:

• add meta box – custom field : which to choose?
• How to add option box in “Edit Post” plugin API?

Check the comments in the code and also the message that’s printed in the MB.

add_action( 'add_meta_boxes', 'add_meta_box_wpse_75963' );

function add_meta_box_wpse_75963() 
{
    // This prevents the code from going further if not in the correct post type
    global $post;
    if( 'reviews' != $post->post_type )
        return;

    // Check if the other post type has a post with the same title
    // If it doesn't, bail out
    $other_page = get_page_by_title( $post->post_title, OBJECT, 'guides' );
    if( !$other_page )
        return;

    // We are in the correct post_type (reviews)
    // and there is another post in "guides" with the same title
    add_meta_box(
        'wpse_42440_sectionid',
        __( 'Custom Field' ), 
        'display_meta_box_wpse_75963',
        'reviews'
    );
}

function display_meta_box_wpse_75963() 
{ 
    echo "Insert custom field procedures here.";
    echo "Also create the appropriate <code>add_action( 'save_post', 'callback');</code>";
}