Determine if a meta key is set for a given object

meta-exists.php

<?php

/**
 * @link https://developer.wordpress.org/reference/functions/get_post_meta/
**/

// Only for post types
if( get_post_meta( $post_id, '_meta_key', true ) ) {
    // do something
}


/**
 * @link https://developer.wordpress.org/reference/functions/metadata_exists/
**/

// Check and get a term meta
if ( metadata_exists( 'term', $term_id, '_meta_key' ) ) {
    // do something
}
 
// Check and get a post meta
if ( metadata_exists( 'post', $post_id, '_meta_key' ) ) {
    // do something
}
 
// Check and get a user meta
if ( metadata_exists( 'user', $user_id, '_meta_key' ) ) {
    // do something
}