Answer:
Sometimes the form factor can be determined without opening the case and the form factor can be predicted on the basis of the shape of the case.
But, since the mother board is must to determine a form factor, you have to open the case before you can determine the form factor. Unless the manufacturer tags the details of the motherboard outside the case which is very rare to see. Also most cases easily show the type but that is not enough to determine the form factor.
