When you have an even number of bolt holes you can measure from center of one to the hole directly across from it. See photo above.
For an odd number of bolt holes, measure from the center of any hole to the center halfway between the two bolt holes directly across from it. See photo above.
Measuring the distance (D) from the center of one bolt hole on the wheel to the center of a bolt hole right next to it can also help to determine the bolt pattern.