Your first alarm warning to walk away is the first time payroll is missed. Unless you have some personal attachment to the founders or summat, no amount of "vision" and "passion" is going to make up for the fact that the company is doomed and/or you're being scammed.
If you're not on payroll, as another commentor mentions: get money up front. If that's a problem, walk away. Take a page from my patented School of Hard Knocks: if the relationship starts with money problems, you're not suave enough to salvage it. (Source: almost ten years running a consulting company.)
I think people miss this because of the Sunken Cost Fallacy.
I've been freelancing 8 years and I can't imagine taking on a project without 50% upfront. The rare instances I missed that, I've lost money -- fortunately not more than a few thousand total.
If you're not on payroll, as another commentor mentions: get money up front. If that's a problem, walk away. Take a page from my patented School of Hard Knocks: if the relationship starts with money problems, you're not suave enough to salvage it. (Source: almost ten years running a consulting company.)