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This is not a sure bet, but just ask about their latest funding round and how much they got and how much of a percentage they gave to the investors. For example, Company X just closed a 1 million dollar Series A and gave up 50% equity. That means the company is valuated @ $2 million.

You can request an offer that specifies your stock options as a percentage of the company. You won't know what it's worth now, but 1) honest founders will tell you, and 2) either way, you'll be able to do the math on what it would be worth in varying exit scenarios.

Aside from asking about internal company details such as financing valuations, you should also do research on similar companies that are a bit more mature. Find out details on their financings, revenue, exits, etc. It'll give you some idea of what could happen.