Whenever trying to get information about any significant factor in a new job, I like to take a multi-step approach:
- Define what that thing actually means to you, in a practical way. "Work life balance" is so generic, and so abused as a workplace term, that it is not meaningful on it's own. Write down the actual factors that you care about. The good news is, you've already done this step (you care about too many nights or weekends).
- Evaluate the job description. This may seem obvious, but it's surprising that some candidates miss things written right in the job description. Does it mention things like production support as needed or other tell-tales that you'll be required to work on demand, and perhaps after hours?
- Research the employer. This can require some digging, but check your LinkedIn or other social networks to see if you know anyone who works there. Look on job review sites for intel on that employer. Consider the broader picture, too - an employer that's supporting a 24/7 business model (like, a bank) or one where client deadlines are critical to success (like, a startup) might be more apt to need extra work from time to time.
- Use specific questions to fill in the gaps. Interviews are a two way street: the employer evaluates your skills to see if you're fit for the job, and you evaluate the employer to decide if you'll be happy working there. Take advantage of this - prepare questions about things that are important to you.
Your question is really asking about that last point, but I think it's worth describing the whole process so you can ask questions in context and not in an isolated manner.
Which brings us to your actual question: how, exactly, do you word your query?
Generally, you want to be specific, but you don't want to leave a negative impression on the employer. You also want to make sure that you're asking questions that they will give honest answers to - some (devious) employers will try to give positive answers to questions that are clearly very directly asking about a sensitive subject. Here are some questions you could try with all of that in mind:
Can you tell me about your company's culture, and what makes it unique? Why do people like working here?
That lets an employer focus on positives. If the company excels at work-life balance and doesn't constantly pressure employees into working extra hours, you will usually hear that when hiring managers answer this question!
Can you talk about the work rhythms for this position? What is a typical day like? Are there any unusually busy or slow times of the year?
Similar to the above, this is open ended, and lets good employers show off why they are good. If the hiring manager says, we're always busy! or something similar to that, you might want to take note - especially if they don't really provide any additional explanation.
Some people advise being extra-careful when asking questions, in the interest of not upsetting the hiring manager or causing them to think you're not a hard worker. While it's true that you don't want to deliberately sabotage yourself, consider also that sometimes, not getting the job offer is a good outcome. If an employer is upset that you want work life balance, do you actually want to work for them?
Finally, in addition to asking questions and taking the other steps described above, you can consider observing for contextual clues. If your interview is around lunch time, are employees actually stopping and taking a lunch break? Does the office have a cafeteria area, and/or obvious spots for breaks? If you drive by the office at 5:30 PM, is the parking lot still just as full as it was at 10 in the morning? Worse, is it crammed full on Saturday, too?