When DS was in first grade, and I explained to his teacher how high he tests for math, I could tell that she was thinking that I had just hothoused him in math. He learned basic fluency, math facts, was taught the steps to do calculations, etc. But then she worked with him and realized that he is actually gifted in math. You want to look at how he does with word problems, and concepts that he has not been directly taught. For instance say you haven't taught him anything about ratios or proportions. You tell him that it takes 2 tsp. of butter to make 24 cookies, how much butter do you need to make 60 cookies? Kids who have been taught ratios would know what they need to do, but gifted kids would just know the answer without knowing what a ratio is or how to do the computations. Anyway, it's just an example, obviously you can't figure out if a child is gifted based on one problem. DS did the Woodcock Johnson Achievement and for math it breaks it down...applied problems, fluency, calculation, for example. DS scored lowest in fluency probably because he is so slow with writing.
Just to note--this test seems to give artificially high scores to younger kids who have learned multiplication, division, etc. and I wouldn't bother with it unless you need the scores to apply for DYS or some other program.
