Are you planning to hire them on as employees, or just gather rent from them as contractors? There’s a little less pressure on you if you engage contractors, but the trade-off is that they’re more autonomous.
Either way, I think it’s important that your prospective trainers seem like a “good fit” with your business ethics and your members, because they’re going to be in your space day in and day out. Someone who has had experience training the types of group-ex members you already have will be a bonus to both of you, because they’ll be able to sell training to your group-ex clients.
I look at resumes, but I also like to see client reviews so I know not only that people like them, but WHY they like them. A few client reviews can give you a great picture of who a trainer is. If I were a studio owner like you are, I would also look at how many clients they have on their current client list that would move over to my space. What type of clients, and can I handle that load of volume, and would these clients be interested in my group-ex classes (win-win)?