Who are their board members?
Who are their investors?
Why did the cofounders choose these people?
What is their runway (hint, if they say they have three months or less of runway, think deeply about the risk you are taking on)
What is the tech cofounders background in? Does he have technical leadership skills? (Trust me you do not want to join a startup where the tech cofounder is barely able to write code and thinks design patterns only apply to UI)
Figure out how much equity you need based on how much salary you're giving up to join these guys (I wrote a blog post about how to approximately calculate this [1])
Have they done their research on their market? Is their market willing to actually pay for a product? Basically, how do they plan to become profitable?
[1]: http://dhurley14.github.io/blog/2016/02/21/how-to-equity/