We test for impressions. In a very recent case, a lay member of a High Court tribunal applied for a job to the company that was giving expert evidence in a pharmaceutical case. She was removed for apparent bias. She had merely created the impression of wanting to join that firm and an impression that she might favour its evidence. Look, we all have our prejudices, opinions and preferences.
Judges tend to be private and shun publicity to avoid blowing the gaff on some inner feelings.
Arbitrators sometimes play the same trick. As for adjudicators, stifling feelings, observations or thoughts that's preposterous. Just be normal. And if someone characterises that behaviour as bias, well, maybe the test should be modified since adjudicators are not judges, nor arbitrators nor are they conducting legal proceedings. And if you think all quantity surveyors are paragons of virtue, say so.