I just went though this (I wound up buying a 2015 car) and I feel your pain. It is very difficult to try to figure out what options the cars have when they are at a dealer or even in some cases cars owned by a private seller (although most of the private sellers I talked to were a bit more knowledgeable about their cars). It makes it even worse that Tesla has changed the features and options quite a lot over the past few years. For example, the power liftgate (which was one option I wanted) originally was part of the tech package, but then partway through 2015 the tech package went away and you needed the premium interior package to get the power lift gate. And now, as of two days ago, the power liftgage is a standard feature.
The things that helped me are this... first, figure out which options you want, then figure out how to identify those options. For me, for example, autopilot was a must have. It helps to know that autopilot was introduced at the very end of 2014, so that automatically ruled out any 2012 and 2013 cars. I was also told by a service center that autopilot was added around VIN 50000. So if you are looking at a 2014 car and the last 6 numbers of the VIN are less than 50000 it definitely does not have autopilot. If it's over 60000 then it almost certainly does have the autopilot hardware and if it's between 50000 and 60000 then there is a chance that it has it. Anything built in 2015 and later has autopilot hardware on the car. There is a chance that the autopilot feature was not enabled, but as long as it has the hardware then you can have the service center turn on the feature for a price (I believe it's around $2500-$3000 for an AP1 car, but don't quote me on that). So if autopilot is important to you then even if you aren't totally sure that it's been enabled you can at least know that it has the hardware and can be enabled after the sale. You can also tell if it has the autopilot hardware by looking for the camera above the rear view mirror. Look at some pictures of a 2012 or 2013 car, which you know doesn't have autopilot and then some pictures of a 2015 or 2016 car with autopilot and you can see the slightly different shape above the rear view mirror when looking at the front of the car from the outside. I got pretty good at identifying cameras from fuzzy front shots in ads.
Other things are a bit easier to identify. Like with the power liftgate for me, it's easy enough for the dealer to go out and see if the liftgate opens and closes itself (although it's important to specify that you want it to close itself. I had one conversation with one dealer who told me a car had a power liftgate because he clicked the remote and the hatch popped open, but that was just what the standard non power liftgate does). With the cold weather package the car has heated rear seats and the standard car only has heated front seats, so if the cold weather package is important then ask the dealer whether it has heated rear seats. The premium audio package has a sub in the trunk, so if you want that, ask whether there's a speaker on the side in the trunk, etc.
One thing I will note is that as near as I could tell cars built in early 2015 and 2014 did not have turn by turn navigation without the tech package. In the middle of 2015 when the tech package went away then turn by turn navigation became standard. So if you want turn by turn navigation and you're looking at a 2014 or early 2015 car then make sure it has a power liftgate as that was also part of the tech package.
Also, ask if they have the original window sticker. For me most of the private party sales did and about 50% of the dealers did. The ones that had it were happy to send me a picture of it.
Finally, I had mixed luck asking service centers to tell me what options cars had. I tried not to call too often because I didn't want to bother anyone, but in a few cases when I had identified a car that I was very interested in and I just wanted to be absolutely certain about some options I tried to call. I simply stated that i was looking at buying a used model S from a dealer or a private party and they weren't sure what options the car had. With the last 6 digits of the VIN the service center is able to identify the options the car has. But from the calls I made it didn't really seem like they had a list of options they could just read off to me. Instead I had better luck asking "does this car have autopilot enabled and the tech package?" A few service centers told me to call the 800 number and both the 800 number and a few service centers told me that they couldn't give me any information about a car that I didn't own because of privacy concerns.
If you let us know what years you are interested in and what features are important to you we can probably give you some specific tips.