The evaluations we were given from our son's PreK teachers were things like: Can Identify All Letters, Can Count to 100. Can Write Upper and Lower Case Letters, etc. They did not evaluate anything beyond what they considered very good for most preschoolers. Of course they realized he could read and knew an awful lot, but they didn't much care to challenge him. Eventually we got a Gifted and Talented Consultant in to do some grade level testing for him in order to see what he was really capable of. This was important because now everyone understands what type of work he can do and we're all on the same page. He is doing a grade skip with subject acceleration next year. Eventually the school may have requested an IQ test, but we decided to do it on our own so it was on our time table. We found it helpful to know the score so that we could have a better grasp on what sort of accommodations he might need. It was also an eye opener because he scored higher than we thought (feared?) he would. I think it was good information for us to have, and it was useful for application to DYS as well.
My son had some periods of struggle during PreK. He became aware of the differences between him and the other kids. Sometimes he enjoyed their activities, but he often acted out when he wasn't challenged. Your son may not do that depending upon his personality. You could talk to the K about early entrance (if that's a possibility where you are) and find out their general policy, and then you should also talk to the preschool teachers and see what their classroom is like and if/how they would be able to work with your son at his level.
Some ability testing may be helpful for you to make your decision, but ultimately much of it will be his personality and your feeling on what will be the best fit for him.
