There are many reasons that a car might pull to one direction or the other. Some simple troubleshooting techniques will help car owners to diagnose the exact problem.
First take the car for a drive on a straight flat road. American road will have a slight tilt to the right to help the rain drain. Also most American car’s alignment is set to drift slightly right as well. This is a safety feature so that the car will tend to go off the road rather than into oncoming traffic in case the driver loses control. So if the car pulls slightly to the right that is as it should be. If it pulls left or severely to the right there is a problem.
Next take the car to a place to add air to the tires. Check the air pressure in all the tires. If both the front tires are not roughly equal in air pressure, say within 2 or 3 Pounds per Square Inch (PSI) then air the tires to an equal amount and take the car on a drive over the same stretch of road. If the car now drive relatively straight; problem solved. Make sure to check the tire pressure regularly.
Speaking of tires, if the car owner has recently changed one tire on the front and not the other, or the front tires have different tread designs then this can cause a pull to one direction or the other. The way to fix this is to let the new wear off the new tire or get matching tread designs on both the front tires.
Next, park the car on an even and level pavement from front to back and side to side. Stand about ten feet away and look at the car’s front end. Does one side of the car seem significantly lower than then the other? If it does, go that side of the car and push it down, does it bounce back slowly if at all? If the side seems low and does not bounce back as it should under pressure than this could have a strut (on front wheel drive cars) or a shock (on rear wheel drive cars) that has lost pressure and needs to be replaced.
Lastly if none of these fixes have worked take the car to a trusted service shop and have the technician put it on the alignment rack and check the castor, of the three parts of an alignment; camber, caster and toe, only caster effects steering. If the caster is off, it will cause the car to pull or drift to the side with the less positive number.