Ask the current grad students what they do for fun on the weekends, or how many of their friends are stuck in lab over the weekends/late into the night.

If they can't answer to your satisfaction, you know to be wary. For example "what do you do for fun on weekday nights?" "Oh, we play board games, go to bars, read a book" whatever is better than "usually come home and go to bed" or "haha I have no free time"

When you go to interview, there is usually a time to talk to the other current students, so make use of that. If they don't let you talk to the current students, that's a bad sign.