Keeping track is an odd term. It suggests you know in the moment what you learned and then forgot. That's not key issue when it comes to measure meaningful learning.
The key feature of real knowledge is that it can be used to achieve desired goals in the future. If you there are no possible goals with which information that you studied helps you, then you haven't learned anything.
While I personally don't want to invest the necessary effort to aquire a foreign language, they are good examples:
You want be able to speak French while taking a vacation in France without needing to switch language. This is testable by actually taking a vacation in France.
It's also testable by taking a CEFR language level test. A friend once told me that while studying a new level himself he took all the CEFR test for each level. This allowed him not only to know when he had enough knowledge to make it in France but also measure his progress.
Measuring progress this way is useful because it tells you whether your learning approach actually works. If you for example just use Anki with French cards, you might feel like you are making progress because you learned more Anki cards without actually moving to the ability to speak French. Using the CEFR language level test on the other hand actually tracks your ability to use the language.
Good measurements differ from subject to subject and require thinking about about what matters that's easy to access in your topic.
It's worth noting that assessing your own learning is far easier in domains where there are practical tasks gated by knowledge. E.g: When learning.a programming language, I can measure the learning by my ability to do tasks of increasing complexity with it.
I imagine that for textbook learning you could try exams. That certainly works for maths although it has a failure mode in that it only verifies that you've memorized passwords whereas what you want to do is to develop a deep and intuitive understanding.
In many domains you could answer StackExchange questions as a practical task and you will likely get feedback when you get something wrong.
Do the thing that the learning was supposed to enable you to do. Introspection and reflection look at the work you put in, the resources spent, not the thing they were spent for. You can only observe where it got you to by doing the thing itself.
In mathematics, do the exercises. Do variations on the exercises. Prove the theorems you learned, without reference to the books.
If learning a language, use it: have conversations in it, read it, write in it.
In statistics, analyse published papers as if you were a referee vetting them for publication.
In programming, write software. Software that does stuff you want done rather than toy exercises.
And so on. Notice how well you are doing all this. That is the test of what you have learned.
My personal way of measuring my knowledge gain is rather simple, but I am not sure how obvious it is. I write down definitions, arguments and examples of everything I know. Then I change them if I encounter something related (I track these changes with a diff program). And if a concept has grown from a small list of properties to lots of examples with elaborate descriptions, then my knowledge has grown too. Some problems include categorization issues, finding the best way for referencing sources and permanent media management.
For me, skill progress is more difficult to track because the metrics for each skill seem to be so different for every new task and I am not sure, if some skills are related or not. For example: If I can program a loop for changing a list of strings, am I able to use this skill to program a loop for changing a list of other data types? I would say no, because I have to know how I can change the different data types first. I don't like this, so I mostly do not bother to track my skill progress.
I am not convinced that learning progress can be measured objectively because knowledge and skills are only useful in regards to a specific lifestyle. Another aspect of this: If a group has a specific lifestyle, using the metrics of that lifestyle to determine your progress would be beneficial for you to fit in. Anyway, it is still nice to share different approaches in tracking your progress towards your own goals.
I am not convinced that learning progress can be measured objectively because knowledge and skills are only useful in regards to a specific lifestyle.
Everything can be measured objectively. On the other hand some measurements are useful because they give you valuable information and others aren't.
I don't know if the educational system's way of trying to measure student learning is any good. But it faces a tough challenge! It can't look directly into your mind and see the knowledge stored there. It can't surveille your activities and see how much you've been studying. Instead, it tries to measure learning with grades, and then tries to see if grades correlate with achievements later in life.
But individual students do, to some extent, have access to this privileged information about how they're studying and what they know. They might deceive themselves in any number of ways. But I would expect that most students have ways of assessing their own progress in learning, and beliefs about how accurate and useful their self-assessments are.
For example, students know the extracurricular articles they've read, and the conversations and projects they've participated in. Students can introspect about their own mind, and reflect on the study activities they've undertaken, like flashcard review, taking notes, or reading. Students have a level of self-awareness about their confidence level and raw ability in solving novel problems.
Do you have any non-obvious ways of measuring your own progress in learning? How much faith do you have in your own self-assessment? When you're reading and studying, how much do you think of the activity as "building a new skill," and how much of it is a transient test or a challenge, like a game?