When using toe plates, what is the easiest way to make sure the toe is 'centered' or 'not dog tracking'?

 

You can't unless you have the centerline of the car marked and are measuring each wheel from that centerline. Toe plates are quick and easy to check toe, but actually adjusting it should always be done from the chassis centerline IMO. If toe isn't being adjusted from the centerline then you can't be certain the thrust angle of the car is zero.