Rather than define learning disabilities by their causes, I will rather describe them by their manifestations: how can we recognize them? Assessment can be as simple as looking at spelling errors, and as hard as trying to assess whether your student is a sequential or spatial learner. However, you do not need to know complicated terminology in order to do an informal assessment, to realize that your student might indeed have a different learning style than others you have encountered. For the most part, informal assessment is about patience and trial and error.
Many students who have mild LDs will have found ways of compensating without ever realizing that they have made these changes. You will generally never know about these students' learning differences because they do not exhibit the same "evidence" in their writing. On the other hand, students with moderate to severe learning problems will probably not have been able to compensate themselves and their problems will manifest themselves in their writing samples. The following is a general description of external "signs" of LDs. For a more detailed list of "textual" symptoms, see attachments 3 - 5.