If you bought it yourself or know the person you are buying it from and remember when they bought the machine and know for a fact it's been sitting in their house the entire time. Other than that, it's all in the condition.

I have a Bally Harlem Globetrotters that is supposedly HUO. Can I prove it? Not really but it is virtually mint and still had the plastic protective coating hanging on the side rails and coin door which are very strong indicators. Of course it's HUO.

I have a few others that are supposedly HUO. Put it this way. They are either HUO or they were put in commercial locations where nobody ever played them.