Have a discussion
Just talk to him or her. Tell him you are concerned about cushing's and woud like to be evaluated.
Most physicians would have no problem proceeding with the workup if requested by a patient. Show him symptoms or signs you currently have that could be caused by cushing's. Remember, the first tests are bloood/urine
tests, not mri
. Worst come to worst, get a second opinion if the first one refused.