I'm going to assume that you're just using straight axles, without any castor, camber or toe-in. In that case, the easiest way is probably to just put rulers along both sides of the car, in contact with the wheels. Measure the distance between them at both ends to make sure that they're parallel, then adjust the wheels so that their edges lie flat along the rulers.
Alternatively, you could semi-tighten the wheels, so that they can be moved around with a bit of effort, then manually push the car along a straight path for a few feet. That should force the wheels into alignment, and you can then finish tightening them.