Tires lose about one psi a month on their own, and more when the weather turns cold. A tire can be twenty percent low and still look completely normal from the outside.
Running underinflated makes the tire flex more than it was designed to, which builds heat, wears the shoulders and costs you fuel. Overinflation is not the answer either - it wears the centre of the tread and gives a harsher ride.
The correct figure is on the placard inside your driver's door, not the number moulded into the tire sidewall. That sidewall figure is the tire's maximum, not your vehicle's recommendation.