How do I determine the value of a car?

There is criteria. It's called put it up for sale and see what you get. There is no other way since the market is never the same and every car is different.

You are asking the impossible. Put thing on Ebay or run it through an auction. Whatever it brings is what it's worth.

If you are not wanting to sell it, then there's no reason to worry about what it's worth. Pretty simple.