With neuroscience, they have been able to map the brain using MRI and study the response of the nerves to learning. For example, neuroimaging has been able to show that the parietal lobe of the brain becomes active when solving mathematical problems but these regions show weak activity in children dyscalculia (maths learning disabilities) [1]. This would go a long way in helping early detection of learning disorders.

The cost of the examining a child for diagnosis would be high and in poor countries the cost of the technology itself would make it out of reach to many. Secondly there is a tendency to overlook the environmental impact of learning and focus on the brain (individual). Third point, any chance of misinterpretation of data?

[1] Price, GR; Holloway, I; Räsänen, P; Vesterinen, M; Ansari, D (2007). "Impaired parietal magnitude processing in developmental dyscalculia". Current Biology. 17 (24): R1042–1043. doi:10.1016/j.cub.2007.10.013