Validation trigger

You give this example for field validation for a numerical or currency field - Can you give an example for a related list field that is blank? Is it possible to run validation and return an error if the related list field is blank? If not, can you also give an example for a blank text field? I tried {!field} == "" but it does not validate the formula. Could you also provide an example if you want to validate multiple fields. Thank you. if ({!amount} > 100000) return “Order is too large: {!amount}”; else return null;