I would suppose the age of the machine will depend on whether to trust it or not. I have one from 1947 & one from 1949 and all the wiring looked great, so I never gave it a second thought. There has been no smell or sparking. One I did have to buy a new foot pedal/plug but that was because it was missing and I knew that when I bought it. I was able to take one with me to test the machine.
Before you buy one request to plug it in and test it. If they don't allow you to do that, the price should be drastically reduced. Then I would be more concerned about the motor than the wiring.
Happy hunting!