Also contact a Service Center and talk to them about the car. When I was buying my Roadster they were able to pull up the car (need the VIN, of course), and review its repair history (at least as far as Tesla's service was concerned). Not foolproof, but good for peace of mind.
I also endorse the earlier suggestions to get the vehicle logs and pick through them. USB stick with a directory named "VehicleLogs" (watch the upper case letters). Stick it in the USB slot under the dash (not sure exactly where this is on the 1.5). Wait for the blinking to stop; takes a few minutes for the 8-ish mb file to download. Besides the battery health, you can also get an idea of other systems, especially the various fans. I ended up with a bit of PEM repair work to do that, in hindsight, were in evidence in the logs (error codes 1144, 1146) going back some 6 months.
Also in hindsight, I'd still have bought the car! Just nice to know what I was getting into...