Compentency aside, I think the only real test is on the job and under fire. My take is you'll really have to settle for verified credentials and compatiable personality keys. The response you recieve from employees will be the response you illicit. If an employee perceives that you are open to debate, the employee will likely speak up if they see something questionable. If your response is defensive, curt, or non-receptive-- or if there is a debate and you are proven correct, but become condescending in response to the attempt to debate, there will be fewer and fewer future debates.

Work relationships are developed over time. It's unlikely you'll be able to determine what a future relationship will be during an interview and hiring process. We all are on our "best behavior" during the interview and hiring process. If you're not getting what you want from senior, qualified people then look carefully at your interactions with these people.