6" ruler. With the wheels off, line the ruler up with the kingpin tops across the line of the axle and then check each axle by eye. I usually do this with the electrics on so the servo is held at its centre position.
The eye is astonishingly accurate! It's easy to see if the axles are parallel to the ruler. It's also easy to add a smidge of toe, in or out, equally each side. HTH
