I was just thinking...one thing that I use to track DS7's progress in math, is word problems. I find difficult, beyond grade level word problems (Singapore is my favorite right now), and hand them to DS to solve without my intervention.
It sounds simplistic, but it really gives me a lot of information about DS's thinking. Singapore is very analytical (I'm sure there are other great math curriculums). Some of the word problems, even at the fourth grade level (what we're working through) are challenging. It shows me that DS is able to complete about 70% quickly, without any instruction or input, but some of the multi-step, challenging problems require me to suggest strategies.
This may not be what you're looking for, but (at least in math), obtaining a math work book above grade level (something hard like Singapore or the AoPS) that emphasizes solving applied (not calculation) type problems will quickly reveal some strengths and weaknesses.
I think this makes complete sense-- it's like contest math, but the junior edition, as it were.

As for tracking progress ourselves:
we tend to look at samples of DD's work for school-- in her most appropriate classes, or with those assignments where she has worked at HER level (not necessarily at the level of the class or the assignment).
This is how I'm reasonably confident that her analytical ability using the scientific method was at college level back when she was about 9. Given an open-ended assignment, she designed something that had different treatment groups, valid controls, etc. And knew what it all meant, was able to present results graphically (though she lacked the statistics that she would have needed to discuss significance in differences with treatment groups).
In another example, when I look at her writing, there are clear signs of progress-- I estimate that she's well into college level ability there as well at this point.
Sometimes such progress comes in large jumps. Now that we've gotten used to that, and now that DD has the metacognition to 'feel' that happening, we know to look.