Added is_paying_customer() to easily check if a user is a WC customer:

Usage:

`
$customer = new WC_Customer();
$out      = 'Paying customer ? ';

if( $customer->is_paying_customer( 10 ) ) {
	$out .= "yes";
} else {
	$out .= "no";
}

echo $out;
`

includes/class-wc-customer.php

@@ -188,6 +188,22 @@ class WC_Customer {
 		}
 		return false;
 	}
+	
+	/**
+	 * Is the user a paying customer?
+	 *
+	 * @access public
+	 * @return bool
+	 */
+	function is_paying_customer( $user_id ) {
+	
+	     $paying_customer = get_user_meta( $user_id, 'paying_customer', true );  
+		 if( $paying_customer != '' && absint( $paying_customer ) > 0) {
+			 return true;
+		 }
+	     
+	     return false;
+	}
 
 
 	/**