A claimant has a right to a fair hearing. Fair might be construed as impartial. If the ALJ, for example, states she or he does not "believe in disability," the statement could be construed as indicating the ALJ is not impartial.
A claimant has a right to obtain an attorney. Some claimants obtain an attorney very close to the date of the hearing. Often the attorney is already scheduled to be in another legal proceeding at the same time. In that case the attorney might advise the claimant to attend the scheduled hearing, advise the ALJ an attorney has been retained, and request a postponement of the hearing.
If the claimant follows these instructions and the ALJ responds, "Why do you need an attorney? I've had your file on my desk for two years and I've already made up my mind," this statement could be construed as indicating the ALJ is not impartial, or non-adversarial.
The Social Security Administration has a policy of recording hearings. In some cases there is conversation in the hearing room between a claimant and ALJ or the claimant's attorney and the ALJ before the formal hearing begins. Sometimes there is also conversation among them after the hearing ends.
A claimant and/or attorney may request the hearing attendant begin recording as soon as the claimant, or claimant and attorney enter the hearing room and keep recording until they leave the room. Ultimately, the decision when to record is the ALJ's.
Assuming the recording is not altered, the claimant, and/or claimant's attorney, and ALJ have evidence should there be any question regarding impartiality or misconduct.