My family has experienced 3 math accelerations. Each one was different in what sort of information was used to assess the acceleration, including informal state standardized test for the year above, KeyMath, WJ achievement, curriculum-based assessment (end of year test as published from the math curriculum), and "meh, let's try two grades up."
I personally find that an 80% or more on the curriculum-based assessment and then patching the gaps on topics missed to be the most satisfying of the three processes we saw. This is also an evidence-based approach. Check out Developing Math Talent by Assouline for more details on this.
