You could get a good estimate of this just checking Crunchbase. Assuming the company is doing well, none of the weird stuff like ratcheting kicks in. So just look at how much money went in and what valuation, apply dilutions for later rounds, and calculate what percentage you own by the time you get to an IPO.

This method still isn't perfect (employee option pools being adjusted will not show up, as well as other possibilities), but it'll definitely give you a rough estimate.