Does the CNM you're interested in have a website? I would look at insurance info on the website if so, or even try calling around to see if you can find one. You can search on Tricare's website for providers as well. Make sure do the network and non-network provider searches to get more options. You'll pay less with network providers, but will probably get more options with non-network ones.
For me personally I had just researched OB's in my area and reviews for them. I saw great reviews for this OB office, and then checked their website and saw they accepted Tricare. They are a non-network provider but are worth the little bit of extra money for me without a doubt.