Karel,
To address your question about how you know if your child has a learning disability or not, I can answer because my daughter does. She was in Kindergarten her first year, and was acting "immature" the teacher said, and it was concluded to hold her back. My daughter repeated her second year of Kindergarten...Finally about in October of 2005 I decided to get an independent evaluation on my daughter that my husband and I paid privately for. I noticed things with my daughter that my younger son never did, and being a former child care provider I knew something was wrong. I continued with the evaluations until it was determined my daughter had sensory integration disorder, and an auditory processing disorder. However, the school began to tell me she was ADHD. If she had been placed on this medication, my daughter in no way would "get better" and would have continued to struggle in her learning both social and educational. After meetings with the school, and with our private therapist, the school concluded that my daughter could not be helped to the degree of which she needed. She was TWO deviation below the mean!!! THATS HUGE! She also can hear at 130 ohms whereas a normal hearing level is between 20 - 30ohms. My daughter literally hears a pin drop and it sounds like a hammer to her...So over the course of a day she is totally overwhelmed, and acts like ADHD. Our PS never caught that! She couldnt read or write by the end of her first year of K. The PS has offered to provide support when needed and also some therapy (however, its nothing like what we get privately) and she qualifies just like a regular attending child to get services. We continue with 2 hours a week with private therapy and about 15 hours of therapy at home. While it is a HUGE undertaking for a homeschool parent it has worked. My daughter can now read, completed her 1st grade math book, has had many unit studies this year, and is happy! She was SOOO depressed at school when the kids laughed at her, and called her names. Now with the therapy and ability to have a very quiet learning time, she has grown by leaps and bounds. Another way to discover your child has a disability is by taking the IOWA tests. It gives you an average of where a child should be, and lets you know what needs to be done. I think in general we all know children should be reading by 8, they should be following their peers in traditional school with first grade math books for 6 to 7 yr olds....etc. Another aspect of a homeschooler noticing these things is to be aware, read information, and stay connected with other people who might notice something. HTH
BTW, we get more learning done in one year because we do not take time off for vacations, we school year round (we take only 2 weeks per year). Most kids ahead in their grade probably do the same thing. It also helps prevent those learning lags in the beginning of the school year where most teachers have to go over everything again.