Dyslexia is diagnosed on the basis of a discrepancy between IQ scores and literacy (can be reading/spelling/written work)scores often with other specific areas such as short term memory and processing speed being identified as weaknesses as well by the tests.
There should also usually be evidence that adequate teaching has been provided so that difficulties are not attributable to lack of expreinece/exposure.
No true IQ test relies on verbal or spatial exclusively as some one suggested earier. Subtests for these, and non verbal reasoning are given equal weight.
There is no accepted defintiom of dyslexia, at last count there were approximately 180 different defintions being used by various organistaions and academics often giving very differing accounts.
The whole brain wiring thing is very contentious, with multiple theories but no conclusive evidence for any of them.
IME dyslexics can present very differing cognitibe profiles on testing. Some are very verbal some are very logical etc etc
It is true that teaching methods that help dyslexics help all children and it is somewhat of a myth that they need somethng exclusive and different. But a diagnosis is often very important to the child and parents to understand the nature of thier difficulties and to plan from there. It can often appear a general problem when in fact it is very specific.
(Iam an EP BTW and diagnose dyslexia quite alot).