Take a spool of thread with you, a piece of fabric, and a pair of scissors. Often there is thread in the bobbin but not on top. Even if you can't plug a machine in, you can often thread it and hand turn the wheel to make sure it forms a stitch. That's my bottom-line on a used machine; if it won't form a stitch, I give it a pass as it probably needs to have the timing re-adjusted (which in my case means a trip to a mechanic, which is not inexpensive here).
If at all possible, ask if you can plug the machine in to make sure that the foot pedal actually runs the machine.
Make sure the bobbin case is there (sometimes it's missing) and that the wiring is supple and looks good (and isn't the old cloth-covered type of wiring).
Vintage Singers are always good. The name does not always mean much, as many machines were "badged" with different names. I have bought newer basic Whites and Kenmores that were fine.