From the perspective of wanting to get paid for your work, you will want to consider the same things that investors do because the company won't be able to pay you if they run out of money. Do they have a product that people will pay money for? That's the key question.
Another is: Will they actually pay you. or does the contract they give you talk about "binding arbitration"? That's a huge red flag of a failing company looking for a way to cheat its employees and is doubly bad for a woman or minority jobseeker since she would not be able to sue them for sexual harassment or racist promotion practices. If you care about the company obeying the law on any matter affecting you, you will want to retain your right to sue them in a court of law.
An employee stock plan might be seen as another sign of poor financial status since companies in poor shape will look to their employees as a last-ditch funding source. However, this conclusion is more coincidental than automatic. Many healthy startups have an employee stock plan and getting into one early can make you rich if the company strikes it big.
You will want to be paid by the hour, rather than a salary, so they cannot work you more than 40 hours a week without paying you overtime.
As for the company culture, you may be able to get a sense of it by simply keeping your eyes open. Is there a dress code or are people in T-shirts? Is everything neat and tidy or do people have toys at their desks? These are things you can see for yourself. As for the core principles, you can simply ask.
You will also want to consider whether you will be happy and productive in the position, and if you can see yourself making a positive contribution to the company and its customers.