How do I find out how much this stock is realistically worth?

So about 10 years ago, during the dot com boom, I was involved with a startup that issued some stock for early employees and seed money investors…

They’re still around and have since received a few rounds of financing and are currently making money… even in this economy.

They aren’t publicly traded anywhere that I’m aware of, and so I’m just wondering how I would determine their worth and how I might liquidate them if I so chose…

Thanks in advance for the help… and yes, I love ellipses.