It is expensive but the best and most indepth is a neurophys test. You can also ask the school specialist to have a woodcock II test performed that will give you a better sense of what you are up against. I'm not sure I would do a online screening site especially if you are going to need accomodations for your child in school.

Has your DC been screened for dyslexia? If so, do you think any of the online screening sites are worth trying? One of my kids has a hard time reading and I suspect he might be dyslexic. TIA.