Look for the characteristics described in this link perhaps https://www.bombay.school.nz/files/6a086d0f68e4a1c0/file_attachments/107/Handout_for_2017_Writing_Sample_Webinar.pdf
You might be able to use the Personal Image Classifier extension to identify upside
down or backwards drawn letters etc. This link provides an example of what might be possible Personal image classifier display
What app is that? An example of something you think could work to help the community provide advice with respect to programming something similar. I doubt that app can do what you think as to rating a dyslexia behavior or have any clinical value. A simple app may indicate possible signs of dyslexia. Anyway, good luck.