Hi ex cm here.
Look at the environment. Is it clean, child friendly, welcoming, bright and cheerful. Look at variety and quantity of toys and equipment. Is there an outside play space/garden. Outdoor toys.?
Look at her pack, most cms have a pack for parents to look at stating all policies and proceedures, hours of work, pay, holiday and sickness proceedures.
Ask about her day, activities, school runs and toddler groups if any, library, soft play, park, walks and indoor activities.
Ask about food, snacks, drinks.
Look at the cm herself, does she provide references or can you speak to present parents or ex parents?
Is she warm, smiling, cheerful. How do the mindees interact with her, are there hugs?
Also hear in mind that the cm is interviewing you as well, it's a two way process and I have rejected parents who I felt I couldn't work with.
Write your questions down, she will be expecting this so feel free.
Good luck and when you do choose take this advice, READ the CONTRACT before you sign as its a legal document that you both have to adhere to so make sure you understand exactly where you stand.
Good luck.