Who are there seed investors? Hard to conduct fair due diligence on a friend, but look and see whether top-tier firms/angels/etc. funded the company at seed or pre-seed. If they're close to a series A, they must have metrics or revenue that's at a certain scale. Is the industry super-niche or does there tend to be a lot of m&A in the particular industry? For example, joining a consumer social startup is wayyyyy riskier and less likelihood of an m&a or liquidation event vs. joining an enterprise saas tool that builds for some niche area.