Often licensing and credentialing rather than the immigration step, because verifications of training and prior practice depend on third parties. Ask the department about its own experience with recent appointments of physicians trained outside the United States.
Sequence the slowest track first
If credentialing requires verification from a medical school or a training programme abroad, that request should go out early, since it depends entirely on another institution's response time. Ask the department which steps it initiates and which the physician must initiate personally. Agreeing a start date before the slowest of these has begun tends to produce a date that has to be moved.
Ask the department to name the date on which each verification request was actually sent, not the date it was planned. A request that has been drafted for three weeks has not started. Where a training programme abroad has closed or merged, verification can require locating a successor institution, and that search belongs at the front of the schedule rather than the back.
Hypothetical example: a medical physicist joining the same department finds that the immigration question turns on whether the duties fit a profession that appears on the USMCA professions list, while the physician's question turns on teaching or research; both nonetheless sit in the same institutional credentialing queue, and the queue does not care which question is being asked. Ask what the department did the last time an appointment slipped, since the answer describes its real flexibility better than a policy statement does.