Totally second this.

Particularly look at:

1 How they treat their customers

2 How they treat their affiliates

3 How they encourage their affiliates to treat their customers.

And how they tread people they consider to be 'below' them, such as waiters, cab drivers and so on. That's a pretty good tell.

My new best signal is if they don't seem to listen during a conversation. They only talk and occasionally pause while waiting to make their next point.

How they treat wait staff in a restaurant is often extremely illuminating.

Since it comes down to "is this CEO a psychopath", most of the classic red flags can be seen quite soon. E.g. charm, lack of history, quick rage.

Not for borderlines, sadly.