Quick check to determine if a WordPress post exists using the post_name (slug) and post_type (defaults to 'post'). Returns the post ID if found, or 0 (zero) if not found.

wordpress_post_exists.php

/**
 *   Determine if a post exists based on post_name and post_type
 *
 *   @param $post_name string unique post name	
 *   @param $post_type string post type (defaults to 'post')
 */

function post_exists( $post_name, $post_type='post' ) {
	global $wpdb;

	$query = "SELECT ID FROM $wpdb->posts WHERE 1=1";
	$args = array();

	if ( !empty ( $post_name ) ) {
	     $query .= " AND post_name LIKE '%s' ";
	     $args[] = $post_name;
	}
	if ( !empty ( $post_type ) ) {
	     $query .= " AND post_type = '%s' ";
	     $args[] = $post_type;
	}

	if ( !empty ( $args ) )
	     return $wpdb->get_var( $wpdb->prepare($query, $args) );

	return 0;
}

Great idea @delputnam!

You can even make this function more liteweight by using get_post_status

<?php 
/** Determine if post exists or not **/
function prefix_post_exists($ID){
 if(get_post_status($ID) === FALSE){
  // post is not existing
 }else{
  // post exists
 }
}
?>

get_post_status either returns said status of post or false if it doesn't exist.

ref: https://codex.wordpress.org/Function_Reference/get_post_status

@cbuchler get_post_status doesn't work if you want to check if a post exists before using wp_insert_post to prevent duplicated posts. Or I'm wrong?

Hello! For this, there is a function get_page_by_path () :-)