Is what you see at headquarters what you'll get overseas?
You can get a good idea of what an employee is going to want in terms of work/life balance on an international assignment by observing what they seem to want in the U.S. office. I think that, in almost every workplace, you can identify who doesn't want to be there late.
If someone insists on a set number of work hours per day, that will often translate into their willingness to put in extra hours on an international assignment.
Obviously, in today's economy we're seeing employees be a bit more flexible. Even so, what you see of them day-to-day is going to give you an indicator of what they'll need and/or expect when abroad.
Do candidates for overseas assignments have an "international competency?"
You should assess their capacity, or appetite, for change. How flexible are they when their underlying values are no longer the same as those around them? Does this person have an "international competency" -- the ability to roll with things, to be flexible in the face of the unknown?
Certain cultures have a different approach to how they manage work and play; some are much more laid back than the U.S., others much less so. Will this person be able to adjust to these differences? Do they have a personal need for a very hard boundary between their work and their personal lives?
Does a candidate's personal family situation preclude an international assignment?
You could argue that this is getting too personal, but when a person is looking at relocating [his or her] family somewhere and ensuring they get the support they need, then it's appropriate for HR to determine whether this particular family unit can handle this sort of disruption.
Finally: Does your company provide adequate support to expatriate employees?
Companies can do lots of things to make the moving process less difficult.
When we work with clients, we'll often do a survey of their expat populations and former assignees, asking about the good, the bad and the ugly. Consistently, one of the most negatively graded aspects is the very poor quality of support they received during the transition process.
Even for a single person, if they're spending the first month of their new assignment trying to get settled, spending time in various government offices taking care of things that were never anticipated ... that's obviously going to impact their productivity, not to mention their satisfaction.
The actual process of getting someone from Point A to Point B, with proper documentation and a livable house, has a lot more moving parts to it than most companies realize. Most expats would be happy to be able to devote the time spent on such issues to their new job instead.