There are red flags that you can easily spot in a job interview or job posting that indicate that the employer wants 80-hour weeks. And who signs up for 80-hour weeks? Naive young people who don't value their time and don't have other commitments, like side projects or family.
It's easier to gauge over the phone or in person. It's often the way things are said that act as a trigger. If a founder works 60-80 hour weeks, it's going to be expected that you will too.