Generally speaking, the most direct measure of preparation for SSA is performance on the end-of-course or end-of-grade test from the specific curriculum used by the district. If there is no year-long summative measure (which is not unusual for elementary texts), one could also administer all of the unit tests for that grade, which takes a little more time, but also has the advantage of pinpointing the exact gaps that exist, if there are any. Ideally, one would use this kind of data to inform curriculum compacting for a student who scored well, but did not make the 95% benchmark (although 80 is probably a fairer measure, since most schools consider 70% to be mastery of the standards for NT students). Meeting the benchmark for mastery should mean moving on to the next level for instruction.
Another (much more laborious) option, depending on the state, is to have the student take a previous year's version of the state-wide test in that content area for the relevant grade to be skipped. (In some states, these are released one to two years later.) If the raw score received is at or above the level considered proficient/meets expectations for that particular administration, then they should be able to move to the next grade-level for instruction. Repeat until they no longer score in the "meets expectations" or above range.
"Should" being the operative word, of course.
