Get some basic understand about how stock options work (grant, vesting, exercise) as well as how tax on options and shares works in your country. You could start here: http://blog.alexmaccaw.com/an-engineers-guide-to-stock-optio...

If you can estimate the value of the company today (e.g. if they had a recent round of fundraising, and you know the terms) then, providing you have access to the cap table (shares and options outstanding) and info on any different classes of shares, you can work out what the shares you would get are worth. Combined with the info on how the stock options work (strike price etc.) you can work out how much they are worth.

This isn't trivial, but it's just math. The first step is working out what you think the company is worth (given fundamental value, and when/how they might exit/IPO). If this yields a zero or low number, then you don't need to do any calculations.