DiosaBellissima nailed the essence of it.
As a rule, you are owed the fair market value of the car; in other words, what you could have sold it for the day before God threw a tree at it. For most cars out there, this is pretty simple to figure out.
Got to cars.com and/or autotrader.com and search for the same year/make/model as yours. Take note of the mileage on each one, and try to stay within a hundred miles or so of your zip code.
Once you’ve got a list, calculate the average price and average mileage–this will give you a baseline “typical” vehicle against which you will compare your car. You’ll also know the high and low ASKING PRICE of cars like yours in your neighborhood. Asking price is an important distinction, typically a seller will come down 10-20% without batting an eye, so keep it in the back of your mind as you negotiate the sale of your car to the insurance company.
Now, look critically at your car as though you were going to buy it from someone else (before God threw a tree at it). Consider mileage, options, physical condition and verifiable mechanical maintenance. How much would YOU pay someone else for the car?
Beware of book values. I’ve found NADA is generally more in line with actual market activity than Kelley Blue Book or Edmunds (I think Edmunds rolls a d20 and applies some kind of associated logarithm to “Save vs breath weapon” for a 7th level fighter, but I could be wrong). Books are good if it’s an unusual vehicle and you can’t find more than a couple others in the market, but nothing beats a good solid “typical” model.
Going through all this does a couple things. First, it might adjust your expectation of what a reasonable value is for your car. Second, if your value is still significantly higher than what you’re being offered, you’ve got sound evidence to back yourself up with.
If you STILL can’t get something resembling a reasonable offer, see if your policy has an appraisal clause and tell your adjuster you’d like to exercise it. Appraisal is implemented in a lot of different ways even within the same company so make your adjuster explain it to you. Worst case scenario you end up coughing up $300 for an appraisal that confirms the company’s original offer–but at least then you’ll know you weren’t getting screwed. Alternatively, it could favor your position. In my opinion, if negotiation leads to appraisal, then the adjuster hasn’t done a good job determining the value or explaining the offer.