The first thing in differentiating between Officiants is to see how they communicate. During your interview, are they speaking about you, finding out about you, or in general focusing on you? Are they talking about themselves the entire time? Yes, there is a place and time to speak about oneself, but briefly. Any interview should focus on you, your needs, your desires, and what will they do for you. The right Officiant should be wholly engaged in your concerns.
Another tip is to check their reviews. Do they have pictures or videos of previous weddings? If possible always meet face to face with your candidate. Personal connections can tell you more about whether you feel comfortable or not, and if you want that person to represent you.