Great question. Not everyone walks into a position equipped with the experience they need. Some are born with it while others will attempt to be someone they are just not destined or psychologically equipped to be.
Really depends on what stage of the game you are in. As a sole employee / CEO revenue / leads are going to be the guiding factor. You'll be able to "feel" the growth more than anything.
Once you start getting more employees, seeing them working together is usually a good indication you are working well. Does growth just seem to come naturally? Do you find it natural to be firm, but fair?
As a self-made CEO for going on 6 years now, I find that my internal dialogue is the best indicator for how things are going. If you don't have a mentor, get one. If you can't find one, at least find someone who is a CEO that you can bounce things off of from time to time that will give you honest feedback. And remember, feedback aren't orders they are suggestions. Not all feedback I've been given over the years (even with the best intentions) have worked out. Learn from your mistakes and good luck!