You are absolutely right that you should do your own due diligence before joining a company.  I suggest you do this through several channels simultaneously.

1- do your research and analysis on the space that the company is targeting, their solution and uniqueness, the competition etc.

2 - ask people that worked with the founders/executives in previous positions and companies about them, their professionalism, etc.

3- try and speak to the investors in the company if you have some connection to them to understand what their thoughts are and why they invested. 

4- ask the founders/CEO of the company the questions directly; what their plans are, why they think they will succeed, etc?