The worst boss I ever had (and he wasn't even the one removed by police! True story!) went through 5 admins in under a year. Almost six if one of them had quit before new years instead of just after.

Chat up the office staff. If they're treated like shit, run. If the founder blows through them, run. You can find some of this on linkedin.

You can search specifically for former employers on linkedin. Consider how many of them there are; there shouldn't be lots for an A round startup.