Probably the most important thing to determine a jacket’s fit is the shoulder width. If the shoulders don’t fit, then you might as well return (or donate) the jacket.
Then you have overall jacket length. Basic rules will state that your jacket should cover your rear and roughly the length of the coat should be such to divide your body in half.
In terms of sleeve length and how much cuff you want to show, ideally you would show around 0.5” of cuff. I prefer to show 1” or more at times.
The waist should be tapered to your torso. This will probably be one of the more common alterations.
Finally, button stance. Modern jackets tend to put the button stance higher up on the torso, but traditionally it should be around your natural waist. Obviously, different brands and designers have played with this idea. If you’re wearing something a bit more traditional and conservative, then I’d follow the conventional wisdom. If you want to push toward something more trendy, then perhaps you could find yourself with a shorter jacket and higher stance. Personally, I have jackets that run the gamut.