You could look at an endless number of factors to determine whether to join a startup, but here are some of my major considerations. I’ve also found that when these factors change (the product is no longer interesting, the growth potential has decreased), it’s likely time to move on. In order:
Do the people you’d be working with make you feel like you’re the dumbest person in the room?
Would you put your career in the hands of the founders?
Would you want to be friends with your potential co-workers even if you weren’t working at the company?
Will this company be fast-growing, but not compromise its values for growth?
Will the growth the company experiences allow you to learn new skills, learn about different markets and become a leader?
If the company succeeds in its mission, does it have a high likelihood of staying independent or selling to a company you’d want to work for?
Is the product that the company has built something you’re interested in working on, contributing to, or selling?
Does the product have the chance to beat existing competitors?
Do you trust the product/engineering team to build something great?
Will you be happy doing the job you’ve received an offer for (and only that job) for at least two years?