If I took on a student from another instructor I would check their folder for the obvious. The lesson, any repeat lessons, and any ground instruction. Some questions I would ask the student are "Where are you in your flight training?" "What have you gone over with your old instructor?" "What areas do you think are your weakest?" I would want to ask his old instructor what he thought of the student. I would also ask the instructor what they have been doing, but would not necessarily do the same as the old instructor, just incase the instructor is the reason for the switch. To figure out where he is on his ground, I feel the easiest way is to quiz him. Good times to quiz him would be walking to and from the plane, during the preflight, and in flight ops before we go out to the plane.