Incidentally, how can you tell dyslexia rather than just somebody who cannot write (a serious Q)?
I don't claim to any great authority, but the few people with dyslexia I've known seemed to have a particular flair for misspelling. I secretly enjoyed one dyslexic friend's emails for their preposterously logical misspellings. Her grammar, punctuation and content all seemed pretty good: if you ignored the spelling they read like emails from an intelligent person.
Perhaps trying to spot that a person has dyslexia isn't actually the right way round of thinking about things. If I get an Email from someone who otherwise seems reasonably bright, I'm always willing to give them the benefit of the doubt until proved otherwise.
Personaly, and like Mark Twain, I wud liyk to see zpelling abolished. I shudder when I think of the amount of time wasted by two billion people learning the difference between wright, write, right, rite and so on...