The driver's seat has a switch inside the buckle, and that is then connected to the seat belt computer. When you start the car without it buckled, it'll beep and illuminate the seat belt light in the instrument cluster for a few seconds.
Also, you mentioned you rarely wear the lap belt. You are probably aware of this, but in case you aren't:
Wearing only the shoulder belt is useless without the lap belt. A person's centre of mass (just under your bellybutton) is well below the shoulder belt, and in a collision the body will "submarine" under and forward from the shoulder belt, usually catching it on the chin and neck. Not telling you what to do, of course, but that's just the facts with auto belts.