Disability support services
The disability support staff at your institution should contact you when you have a student with a disability enrolled in the course you are teaching. They may or may not be able to notify you of the specific diagnosis, as this will depend on the regulations of your institution and whether or not the student has given permission for their disability to be disclosed.
Meeting the student
Once you have been notified about the student's disability, it is worthwhile setting up a meeting with your student to discuss the student's specific support needs so that you and the disability support staff can provide tailored support. If you do not know what disability the student has, the student may be willing to disclose this to you in order for you to provide more individually tailored assistance.