Obviously, you can't predict the future; you'll never know if a company is going to go public at a valuation of billions, or go broke in a year. But you can use certain guidelines: How many companies in the industry have gone public/sold/failed? What were the successful companies' valuations? What's your employer's current valuation? Etc.

Moreover, anything less than a percent of equity in a startup really isn't worth your excitement. Only a tiny handful of startups ever sell for valuations of >$50 million. In the event of a sale at that valuation, your 1% stake is worth (at most) $500,000 -- nothing to sneeze at, certainly, but not enough to retire young. Yet very few engineers are granted that much equity; unless you're one of the first employees, a more typical stake is 0.1% or less. So now, that $50 million exit is going to pay you $50,000 -- a nice down-payment on a house (in Ohio, maybe). About the only way to get rich via employment is through extreme luck (i.e. find the next Google and join early), or through the gradual accumulation of shares over a long period of time.

Bottom line: demand fair compensation, and don't sell yourself out for stock options. Startups can be great places to work, they're fun, and you'll learn a lot, but unless it's your startup, you're most likely not going to get rich.