Its tricky. I usually wait until near the end of the interview, after I've asked a bunch of questions, especially when they say "do you have any other questions?" Then I say, "Hmm. What is work/life balance like at X?" I used to wait until the end of the full-day interview, but many times found out I'd wasted a day on a company that I wouldn't want to work for. So now I do it at the end of the phone screen.