There are few hard and fast rules, there was after all a lot of experimentation and variation within the glass produced. Judgements like this are usually subjective (a little simpler in this case because of the signature) and require the weighing up of various factors to reach what is usually a tentative conclusion. Attempting to judge on a single feature (colour, neck height, etc) will probably at some stage trip you up.
So not an exact science.
John