The way cops know if you're not wearing a seat belt is they look from their vehicle when possible to see if the strap is over your shoulder.
If they don't see that strap over your shoulder then they may get closer to get a better look and in some states police can pull you over simply for not wearing your seat belt.
But in other states a police officer cannot just pull you over for not wearing a seat belt and must have other causes to pull you over first.
Then they ticket you for the no seat belt if you're not wearing a seat belt.
In some states I've not worn my seat belt and the cops looked my way but never pulled me over.
But on some other states I've been pulled over for not wearing a seat belt and ticketed.
So it can vary from state to state as to their laws on seat belts.
Also old antique cars that were never fitted with seat belts are not required to have seat belts so you cannot be ticketed for not wearing a seat belt in an older vehicle that never had a seat belt or seat belts installed.
As long as the old car or other old vehicle was never fitted with seat belts from the factory then you have no obligation legally to install or wear seat belts in the old car or vehicle.
If the old car came with seat belts then you must install new seat belts if there are none and wear them when driving or riding in the car.
Also although the car or other old antique vehicle never came with seat belts you cannot have any kids under 3 riding in the antique car.
If you have kids under 3 or kids that legally are required to ride in a car seat child restraint then you must have at least a seat belt installed to secure the child's car seat child restraint.
But for adults and those who do not legally require to be in a child restraint seat they can ride in the antique car without seat belts.