Generally, the oldest slot machines were significantly smaller than those that're currently being used and were approximately the size of an old cash register. Yet, if you can't find any hard documentation stating the machine's manufacturing date, you can rely on the visual component to see if you can figure out a ballpark date range. Reputable antique slot machine dealers will usually offer a certificate of authenticity with the machine. A real antique machine should have a serial number, unlike a reproduction slot machine. The more original parts that an antique slot machine has, the more value it holds.
