I think it is always quite interesting to have your kid in the room when the interview starts, and see whether they bother to try and engage or whether they just ignore and focus on you, the adult. People who like kids tend to want to talk to or engage with the kid as well as the adult.
I also like to ask them to describe themselves in one word - it's an old corny interview trick, but it puts people on the spot and so you tend to get the truth. Someone once gave me the word "class" and that put me off!
I also ask what experience they have of looking after kids - as that is half the battle.
And ask them OPEN questions - eg what did you last do in work/college where you feel you did a good job, and why. Or tell me something you feel you could have done better and what did you learn from it. it's really just to get them talking freely, so you get a feel for the real person rather than just formulaic CV answers.
Apart from that, it's good to have personal references - and actually ring and chat rather than just rely on written word, as then you can ask the referee open questions too - eg "was there any area you feel x could have improved upon".
The other interesting question to ask is why they want to work with autistic kids - of course they will talk about the challenge etc, but you can usually tell if there is any enthusiasm there or if it's just all by rote.
Also, if they are late, or seem grasping, or lazy, be wary. It's a tough demanding job.
You need to be quite intelligent and flexible to work on ABA, as you need to change the style of working if the kid is not responding. My best tutors so far have been a) previous ABA tutors b) a nursery teacher who wanted to train up on ABA c) a psychology student who wanted work experience and d)a nanny who wanted to widen her skills.
good luck!