This is what I would do. I would have the interview in 2 parts the first would be verbal like going over his or her application getting a feel of the person talking about what he likes about the field that you are in and the next phase I would take this person to a open job site and ask certain questions like how long would you take to complete this job. I would even have a conversation about reading a tape measure etc. the small questions can sometimes be a deal breaker. I have a lawn care business and if was to hire someone I would take them to one of my jobs and ask them how long if you was alone,how long would it take you to cut and weed eat this yard and blow it off. I know how long it would take me to do it. Conversation about your field is a must.