yobro, pop the top (it doesn't require a screwdriver -- just pull it off) and you'll see the stack. If the machine powers on, watch the entire stack do at least one full revolution. (If it doesn't power on, do this with the hand wheel.) Check for any cracks, etc. This is a music box design so there is a series of following arms, one for each cam. This is an interesting departure from the more standard traveling arm. Also, try to move all the dials and see if you can select any/all of the stitch selections in the radio display.
I got mine for $15(? -- or was it $10?) if I remember correctly. I think it's a cool-looking machine. I didn't buy mine for its sewing ability and if I can get it to sew, gravy. It powers on but I'm working on unsticking the entire machine but I haven't really touched it in months.