Instructions
Check to see whether the slot machine runs on electricity. Does it have flashing lights, music, animated displays? Is there wiring at the back? If the answer to any of these questions is yes, then the machine isn't an antique. Antique slot machines were entirely mechanical, replying on a tug upon the playing arm to power them up.
Inspect the slot to see what kind of coins or tokens it accepts. Antique slot machines usually accept only very small denominations such as 5 and 10 cents. Any special bonus payouts mentioned on the face of the machine should be similarly modest--perhaps $20 maximum.
Look at the body of the machine. The metal should be of a thick, heavy gauge, with cast swags and flourishes and applied-enamel decoration. Modern reproductions or later originals will be of a lighter gauge with the weight centered more in the base.
Examine the machine for wear and tear. Reproductions will either look brand-new or the wear will be poorly faked. When checking for authentic wear, think of countless sweaty fingers gripping the handle and forcing coins into the slot, and imagine those coins pounding into the cash-box and release tray. Do you see a pattern of wear that evokes that kind of activity? If the answer is yes, then the machine is probably genuine.
Inspect the machine for a serial number. Any working slot machine would have been issued with a number so that the owner could monitor it. Reproduction machines made for domestic use tend to omit that key detail.