You can see the metal or plastic gears by simply opening the top and bottom covers. The Singer 600 Touch and Sew was about the last of the metal gears. The later T&S had plastic. Sort of confusing because a smaller model number doesn't always mean an earlier machine, so you have to know or research the model. The plastic gears are solely a cost saving measure and are only junk 40 years later after a long life when they start to fall apart. (sort of like myself)
I've always liked the Class 15 action which does away with the bevel gears.
Jon