That's a good question, and one that can be difficult to nail down. It's a bit abstract and can change from shot to shot.
I tend to define a beat as any important storytelling moment. This can be as major as a change in emotion, or, if your shot is a more focused on physical action, it could be a weight shift or change in direction. Look for any moment where you are introducing a new idea.
I've heard that a good test for finding a beat in your shot is to look for a change. This could be a change in expression or a change in direction; if you're working with audio, it could be a change in volume, speed or tone. If a change is occurring, chances are it is a moment you can define as an important storytelling "beat."