Ways To Test iMac G3 Before Buying

I am going to buy an old iMac G3 Over the next couple of days/weeks, and I want to make sure that I am buying a system with no problems. I usually ask for the serial number and good pictures before buying, but I want to know what to do when I physically meet the person I decide to buy from. What should I do when with the buyer to test the system to know what defects it may have? The system would either be running Mac OS 8 or 9 (classic), or OS X. If there is a set of things I should do, or a program I should run what would that be?