Firstly, try to validate the team's dynamics, their shared passion for the vision, and whether their values resonate with yours vision. If you feel you are in sync with the team environment it will be really paramount for your long-term satisfaction.
Secondly, assess the product's market viability, its competitive edge, and the sustainability of its business model. Your role within the organization, its alignment with your career goals, and opportunities for professional growth are equally important.
Lastly, understanding the startup's financial health, equity structure, and your personal risk tolerance is essential.
Try to connect with someone at your level of experience or 1-2 years senior and know about the actual working environment. You can connect over a call through first teaching out to them over Linkedin. I am sure anybody would be happy to tell you about the startup.