"Would you like a box?"
They'll probably ask for one themselves but you could ask if they still have food left on their plates.
Edit: ook, how about "Will you be taking that home with you tonight?" Honestly, most people just ask for a box themselves. What do your co-workers ask? Or ask them "Would you like me to take your plates for you?" and then they'll tell you that they don't want their food or that they would like their food. Or "Are you done with that?" and they will tell you yes or no.