It's not an exact science, but there are plenty of artifacts one would tend to leave around when working extra hours - late night emails, chat messages at odd h

It's not an exact science, but there are plenty of artifacts one would tend to leave around when working extra hours - late night emails, chat messages at odd hours, code commits or comments, whatever. Don't let it become a pattern. Outside of being unhealthy for the individual involved, it can be quite detrimental for the team. Managers especially can set unhealthy expectations without any conscious intent on their part.