I agree with the other posts here that standardized tests + school grades should give you an idea of strength and weaknesses. Our state reports cover various subscores within each subject, and have been quite helpful for us.
However, if your child is already in the high 90th percentiles in the state mandated tests, you can try the following:
You could download a sample ACT test from the ACT website, and give your kid the 35 minute reading test, and then compare the score with the percentile for GT 7th graders using the following document from Duke TIP:
https://tip.duke.edu/downloads/ts/7/summary.pdf
Repeat with the math portion. This will give you a deeper comparison among high performing students in middle school.