Apraxia Diagnosis

My daughter, who turned 2 in July, has a severe speech delay.  I've been doing extensive research, and have read through many discussion threads, here and elsewhere, on the subject.  We have seen a neurologist and are following up with that soon.  We had an MRI and sleep deprived EEG to rule out anything else.  Both normal.  
When she turned 18 months, I could tell there was an issue with her speech.  She shows almost every symptom of Childhood Developmental Apraxia of Speech.  We have had her in ECI since she was 18 months and she has made some mild improvement.  Most of the improvement seems to be more evident in her cognitive growth as opposed to the therapy itself.  She is well above average intelligence and communicates perfectly fine otherwise with her own gestures and sign language.
My concern is how can we know for certain that this is what her diagnosis is?  Does this apraxia show in any other physical tests?  Do you just rule out seizures and everything else and say, well, must be apraxia then?  I am all for treating it as Apraxia and proceeding with therapy, but I'm concerned about how to specifically diagnose it.

Can anyone help me?