There are two main ways to tell if your car needs a front end alignment.
Check the tires for wear and tear. A car that is properly aligned will have tires that will have about the same amount of wear on the treads. If one of your front tires shows more tread wear than the other tire, chances are that your alignment is off and you'll need a front end adjustment.
See if the car 'pulls' when you let go of the wheel. To truly tell if your car needs a front end alignment, drive your car in a straight line keeping the steeling wheel in the starting position and making sure the car is going in a straight line. If you have to turn the wheel one way to keep the car straight, your alignment is likely off.