Take pictures and show us the interior and exterior so we can at least help with the physical bits. I'm no expert on what to look for but I do own one so I can tell you what I would look for.
1. What model is it? 75D, 100D, or P100D?
2. The estimated range at 90% charge.
3. Does it have Enhanced AutoPilot or Full Self Driving package?
4. VIN number for insurance research to see its accident history. If any.
5. Ask if there is anything needing worked on and hope for an honest answer.
There are checklists floating around here on this forum that people use when they take delivery. Here is one that is fairly old, but maybe it will guide you to check out other things!
Tesla Model S Buyers', Delivery and Owners' Guide