In all seriousness, promising startups are few and far between. If you find one (aligned with your interests), by all means jump on it :)
How do you know if a startup is super promising?
i) Check out their source of funding. If super reputable VCs => Good signal.
ii) Check out their attrition. If it's a revolving door, don't go there.
iii) Check out the employees. Best startups tend to have highly credentialed employees (e.g. formerly very successful startup founders).
iv) Is the startup on a continuous growth trajectory? Best startups are on one without much setback. Again, multiple setbacks => Don't go there.
v) I wouldn't bet on the specific idea or the problem being solved. Who would've guessed Facebook, Uber, Airbnb would be massive hits purely based on the idea? Betting on momentum (e.g. user growth) and/or the team is somewhat easier to measure and understand.
vi) Where do you see yourself (and the startup) in 2, 3, 5 years if you join? Will you gain skills and reputation highly relevant to your next steps? How big is the addressable market (and potential market value) of the startup?
It's best to make a list of factors you care about (money, location etc). Rank each option by each factor. Add things up in the end.
These are the rules that I personally follow. Best of luck!