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I just bought a 2018 model X 4 days ago and I charged it twice at the Supercharger just fine even before I created my Tesla account and linked the car. May I assume I have free charging or was the old owner still paying for those 2 charges? LOL
It will say on your account page - sign in, click Manage next to the image of your car then click Details under the VIN number. It will be listed under SPECS. You could also just plug it in to a supercharger.