Some Developmental/ Behavioral Pediatricians test reading decoding & comprehension. Child psychologists, neuropsychologists & commercial programs like Lindamood-Bell do, also. Dr. Sally Shaywitz used functional MRI's to show that different parts of the brain are activated in readers vs. non-readers during reading tasks, but this is for research only. Read her book "Overcoming Dyslexia" for explanation.
Sometimes, and no. A doctor may diagnose dyslexia if given a good description from the parents and a teacher. No x-ray or MRI diagnoses dyslexia. However, dyslexia should be diagnosed with testing by the child's school district (depends only on where he lives). This is required of all school districts in america. Then, parents can pay for another opinion from private testing clinics if parents want more information.