There is an option available in the designer called validation rules. The easiest way to create a validation rule is by using the Validation Rule Editor. The editor is available under the File Menu. This is a picklist-driven interface intended to simplify adding validation rules to your form. The editor supports simple rules like "Field_A must contain ink” to more complex rules like “If Field_B is equal to X then Field_B must not be empty.” The editor even allows for "If…Then…Else" validation rule creation.