Like investing in companies, its not a list of hard and fast criteria. You have to ask yourself if you could reasonably see this company being huge ($10B+) - do they have a great product? Does this have an aha! feel like Stripe and Dropbox did at their public launches? Do the founders really understand their market well? What are the risks - is this a hardware startup in a totally new market? Is the market saturated? How do people feel about the existing products? And do the founders respect you enough to share cap table/preference overhang, the percentage your equity stake represents, and growth metrics? Also, value your equity stake at the current valuation, not a future one - the current valuation already has the expected future growth priced in. What is that equity grant worth per year that it vests? Do you need a larger grant to make it worth the salary disparity vs your other offers?

But basically, you really need to do your homework and treat it like you’re making an investment.