You can tell that the bag you’re carrying around is too heavy when you put it on your front passenger seat and your car disables the passenger airbag.* I guess this is better than the passenger airbag staying enabled and your car beeping because, based on the weight, it assumes your bag is a person who hasn’t buckled the seat belt.
*You know you’re not a graduate student anymore when you can afford a car new enough to disable the passenger airbag based on weight.
Memoirs of a SLACer 