For the auditions, throw the actor a character description and see where they go with it.
If they just sit there with a blank look on their face, then they may be a challenge to work with. That's a start.
For further auditions, do the same but using less exciting, more mundane character descriptions. This helps you filter out those who will only work or want to work if they are "in the spotlight". Not all jobs at a haunt are glamorous parts, and few people can or will carry a single part/character, successfully, the whole night(s) so having people that can and will do multiple parts is a key factor.
Don't be afraid to make comments and teach them, this also tells you how well they will work as part of a team. Again, vitally important.
If you do classes, don't try to jam too much in one class/session, instead, pick a few key skills and work on them 'til they are second nature to your students.
Also keep your eyes open for skilled stage hands/slave labor. If you are going to need security, ticket sellers & takers, souvenirs sales, guides, etc., this is a good time to look for them too. You may find, like many, that you will want people who can cover several positions or tasks, for some, doing these other tasks helps them become better actors because they will know and appreciate all of the steps involved to put on a haunt.