Ranman256's suggestion works. You don't need VBA on each field, though. You can put code in the BeforeUpdate event to test all mandatory fields and use the Cancel option to disallow the update if some fields are not acceptable.
What I did when I had a mandatory field was I highlighted the field's .Bordercolor in yellow and also highlighted the field's label .Backcolor in yellow. Since my form's .Backcolor was a pale green, that made the required fields stand out quite well. And the other side of that is that in the form's Current event, you can reset the borders and backgrounds to fade into the form, either by making them transparent or by making them match the form.
So you would use VBA in two places. One place to highlight stuff, one place to turn off the highlights.