Below is a basic example of how you can assit your form users, by reminding them to fill the mandatory fields. This condition can be applied at various events like, printing, submitting, while filling form, etc. In the following example, validation is triggered when the user leaves the form field.
If a user clicks within the field and moves on without filling it, the following warning shows up.
See this help topic to know more about Form Level Validation settings.
See this help topic to other ways of validating user input.