The diagnostic testing that was done at 2-1/2 years of age involved genetic studies on mom, dad, and child, and a comprehensive evaluation by a number of specialists including peds, neuro, developmental psych, genetics, and probably more that I'm not recalling at the moment. It was a 3 day process carried out at a teaching medical center. The parents were given a list of testing centers and allowed to choose the one they preferred.
It's been awhile since I've talked to Mom, but as I understand it, their insurance paid for the initial genetic studies, and because of that, won't pay for anything additional. I assume that the test they would like to have done is the one that ernurse1973 mentioned above.
They received early intervention services in the home for about a year, until the child reached her 3rd birthday. Those services were quite comprehensive (4 days/week, including speech, PT, OT, play therapy), and Mom was pleased with the progress the child made. At age 3, she began a district sponsored pre-school program, which amounts to fewer hours and less services than she received in the home. The district has limited resources in terms of money and personnel, and I doubt that the child will receive the same level of care that she received prior to age 3. The parents are considering relocating to another school district, but their finances have so far prevented them from doing so.