I am going to be attempting to find the spine of a set of shafts for an older set of clubs I have to learn, I have seen different ways of aligning the shaft spine between either towards the target with the face square or 180 deg opposite target.

How can I easily determine if the face of the iron is square with respect to the target line? I have a industrial protractor set, will this be a good tool to determine? Should the club head be mounted somehow to measure the face squareness prior to installing the shaft?