[nilo]

I will take a controversial approach and will advise you to first set a profile that you would be looking in a captain, thus asking the questions to yourself first.

1- What kind of scope you are expecting from a captain's duty? Do you want him to be a manager for your boat's expenses and repairs, arranging things and handling finances on his own.
2- Are you going to have a number of crew reporting to him and expect him to arrange their duties as well?
3- How is your character in general? Do you use your boat yourself often? Are you detailed oriented and want to get involved in most things about your boat, or do you delegate responsibility?
4- Do you like to be friendly with the people around you or do you like to keep a distance with them?

Etc..

Once you can clarify a profile that will suit your needs than
one can take the technical aspects of running a boat as a must have issues, granted to be possessed by an experienced captain. In this respect, I suggest you check at least few references before you hire.

As to the profile of the nominees for the job, although there are personality tests that are somehow indicating traits of characters, I am sure you can sit down for a friendly interview and have a feeling for most of the points that you would like to have in a person with his reactions and descriptions during the course of your discussion.

If he is somebody who want to socialize easily, then he will not be comfortable if you keep a distance. If he is somebody who want to take responsibilty and you do not delegate, again it will be a difficult environement for him. If he is somebody who wants to work on his own, but you want him to manage a large number of crew, same sad ending.