How do I know what I am settling for is fair?

If you were personally injured (which I assume is what happened, since you entered this question under "personal injury"), then if you went to court and won, you could potentially recover:

1) all your out-of-pocket (not paid by health insurance) medical costs, both to date and reasonably certain projected future (though future costs are "discounted to present value"--reduced in amount, to reflect the fact that a dollar today is worth more than a dollar in 5 or 10 years);

2) any lost wages due to the accident;

3) projected future reduced earning potential, if you can't work as much, as long, or at as lucrative a job;

4) other costs flowing out of the accident, such as if you had to hire someone to drive you or keep house or take care of your children while hurt;

5) "pain and suffering," IF the injury caused a significant impairment of life functions or reduction in quality of life lasting months or years. This is usually (very rough rule of thumb) an amount equal to 1x - 3x your medical costs.

The sum of the above, plus any compensation for destroyed or damaged property (such as a car) is usually the most you can get in a trial.

The earlier before trial you settle, the less you'll get; you get less because you are saving the time, cost, and uncertainty of litigation. Settling before having to file a lawsuit might mean taking 1/4 to 1/3 of what you could get if you won at trial; settling after litigation has gone on for some time, but before trial, you might get 1/2 to 60% or so.