Typically a tutor would not rely on assesments that other formal educators and those in the mental health field would rely on. In a tutoring session however, the student can be assessed formally, but it tends to be based more on informal conversation and questions geared towards determining where the student is at in their current knowledge base and maybe what struggles they have had in learning the material.
With ADHD/ADD students, having something hands on with more immediate rewards helps keep concentration and focus, because with ADHD/ADD, beyond the inability to focus on things that are not immediately stimulating, they have a tendency to hyper-focus on things that are, such as video games or reading, or anything else that interests them, therefore granting immediate stimulation. By creating lesson plans that are hands on, interactive, and giving them breaks to talk about or work on something else, or by creating a lesson plan that they can directly relate to (i.e if they are interested in skate boarding, relate the lesson to skateboarding), you can help the ADHD/ADD student to retain more of the knowledge.
With something like dyslexia, having them practice physically writing out letters can help their brains create connections about the orientation of the letters and in turn helping them start to function with reading more fluently.
With any student, having a routine will help ingrain learning objectives.