I agree with HustleSimmons. Check the battery sticker behind the front passenger wheel. Turn the steering wheel all the way to the right, and you'll see it.
I doubt the seller will let you, but if you drop the rear bottom trim, you can look at the motor sticker also.
I just bought a 2013, so I was in the same boat as you.
Turns out I have the original battery (rev B), but a newer motor (rev Q)
It is a scary financial leap, but I just researched that the original "drive it off the lot" price was $100,000, and I paid $26,000, and it has 100,000 miles on it.
There's a lot of good and bad about Tesla in these forums, but I won't even consider an ICE car anymore, and Tesla has realistic range (same as my ICE car with a full tank of gas) and free Supercharging, located 3 miles from my house.