"Divide their annual recurring revenue by the total headcount. If that number is below $150K, the company is either pre–product market fit or scaling too early."

Most of the time, if you are a technical person joining a company that already has revenue, you are probably already too late for a significant equity stake. And if a company already has $300k of revenue per employee, I'd have trouble calling it a "start-up". Just my experience - I am sure there are exceptions.

Some of the other advice is good - finding out how many shares are outstanding and the current valuation is the info you need to judge what the shares in your offer might be worth. I agree that if the company is evasive on this info, it is a red flag.

Start-ups are risky - but if it doesn't work out, move on. It isn't the end of your career.