Generally they put that in the job description, no? If it's not there, I would move forward without mentioning it, and then bring it up at a later date.

But then, I don't think there's any real downside to asking at the outset.

Absolutely. "Is travel required or expected for this position?"

I am emailing with a hiring manager about a job - is it okay for me to ask about travel requirements? This would be a deal breaker for me.