You might want to contact a group like Parents of Food Allergic Kids or Food Allergy and Anaphylaxis Network and find out if there is a support group in your area. Other parents might be able to pass on what they've learned.
Interview the staff. Ask what they know about food allergies and if they are trained (or willing to be trained) on the use of an epi pen.
Listen to your gut instincts. If it doesn't feel like they 'get it'; try somewhere else.
You'll want an allergy plan in place; and a poster of your child with his allergy and where the medication is kept should be easily viewed by all adults working in the classroom. Make sure that subs and volunteers are informed of allergies as well.