Motors often have a line down the front, you can use that as a start. Then look for something on the dish clamp you can line it up to. Failing that, do it by eye and hope for the best. If its not, you find out its not centrered as the dish won't track the arc, and will be too thigh on one side and too low on the other, so you can adjust to correct.
And equally important is, once you've got the dish lined up with the motor stub datum, is to make sure you tighten the clamps carefully, working a bit on each side art a time, to ensure the dish neither slips nor twists as the clamps begin to bear.