How to vet a startup?

Fellow risk takers, I want to start a dialog about how you might vet a startup?

Hypothetically you are approached by a startup who wants you to join the team. You are interested in the startup and want to mitigate your potential risk by validating the claims being made. What due diligence can be done to ensure the startup is legitimate and has the finances they claim to have? For example, if a startup says we have $X in revenue how can you be certain this is true?

I did some searching and but didn't find any other hackernews discussions.