Any startup should not just plan how to make money, but also test if their idea is feasible. As someone stated, working for startups carry a much higher degree of risk (mostly because you would be working for less $$$ and the difference you are 'investing').

It would probably be a good idea to ask yourself - are you willing to invest in this company? If you are, is it for giggles or a future?

I'd also say if you are looking to join a startup, pick one that is incubated (ex. ycombinator, google umbono etc.)