Many factors have to align for a startup to succeed, but the most important of all is that there is actually market demand for the product. You can have the best team in the world and fail because you aren't making something that someone wants to buy. Conversely, you can make a lot of mistakes and still succeed if you have a product in high demand.

So aside from all the questions you would ask about working conditions, tech stack and financial runway, think long and hard about if you truly believe that there is a market for the product that the startup is making. And likewise, would it be possible to ever make a profit delivering that product?

So many startup ideas are honestly pretty bad - they are chasing tiny markets, they are chasing non-existent markets, or they are chasing low-margin businesses and would have to take over the entire world to be profitable, etc. If you don't completely buy the idea of the company as an eventually profitable enterprise, don't join.

Finally, make sure you get something in return for the risk you are taking - either in immediate pay or real equality. It's so, so common for people to work at a startup for 3-4 years, have the startup "succeed" in the sense that it gets bought up by someone, but in the end all the employees get nominal rewards (say less than 100k) or nothing. That is not worth years of giving up a good salary at a bigger company with better benefits.