I think it's probably different for every manufacturer, exactly when they started going to plastic gearing.
I'm not really positive, but I start getting leery of things built in late 60's. If I see ANY plastic on the outside it makes me suspicious, even just knobs or the plates on the body. I'm to the point now, though - I keep a screwdriver in my purse and if I'm in doubt I'm not shy about opening up the machine to see what's going on inside. I explain what I'm checking for and ask permission first of course - nobody has said no although some have given me weird looks or been a little nervous about it, but I promise to buy it if I break it and that seems to reassure them. People usually trust me - I seem to have an "honest face", lol. (And I brows the local thrift stores often enough that some of the staff recognize me now) A couple times at thrift shops I've gained an audience doing that, which is kind of funny. "What are you doing?" "Are you fixing that machine?" "Hey, my machine is sort of like that and it isn't working, can you help me?"
Or if you have a smartphone with you or are looking at a machine online, a quick Google search might help. Usually you can find pictures of the machine's guts and you can see what's inside.