You'll know if your car needs an alignment. You'll feel pull either left or right. Just make sure your air pressure it correct, that is more of a problem I'd say than alignment.
Actually, in many cases there is no drift/pull present, nor a crooked steering wheel, but both the front toe angles are out of spec (out of alignment), resulting in irregular tire wear. The front end of a car can be slightly pigeon-toed (toe in), or toed-out and if it's fairly equal side-to-side the steering wheel can still be straight, but there will eventually be edgewear on the outside (toe in) or inside (toe out) of the tires.
To the OP, if you don't have any drift/pull and your steering wheel is straight, then just throw the wheels/tires on your car, and check the tire conditions in about 1000-2000 miles. If you notice one of the edges of the tire has a rougher feel, have the alignment checked.