Every five years or so, my wife asks me, "What do you need to learn now for the next 10 years of your career?"
If you're self taught, you can teach yourself the next thing.
You don't have a solid CS background? You can teach yourself that. Go look at, say, the list of MIT courses for a CS degree. You don't have to take the MIT classes. But which of the things do you know, at least to some degree? The ones you don't know, do you feel the lack of any of them? Have they held you back at points in your career so far? Go learn those.
Your work isn't interesting any more? Look around at your company. What work there would be more interesting? What do you need to know to be able to do that work? Go learn that. If things line up just right, you can help out on a project in that area that is short-handed, and get experience on your company's dime. You may have to be able to say "I know a bit about that" first, though, so go learn at least a bit about that.
All knowledge, by definition, had to be first learned by someone. That person didn’t have an exams or someone testing their knowledge.
As a child I looked to exams and grades to validate my knowledge. However I got A’s on exams and felt like I still knew nothing. As I got older I care less about external validation and more about my internal “self grader”.
A few ways you can test yourself is building something. Take your knowledge and apply it.
Or try to teach someone. If you can’t explain the concepts clearly then maybe you don’t understand it as well as you think you do.
Or pretend your the teacher, write your own exams and take them. Ask yourself, “what questions would I ask students to tell if they know something?”
Lastly, and this one I like the least, find some tests or exams online, books that have questions at the end, etc.
Exams mostly don't assess knowledge and understanding, only snapshot regurgitation of facts and snippets. Additionally, when one takes an exam it's rare to require 100% correct to "pass" (and even when that does happen it's only 100% of the sliver of the total subject matter that was assessed).
I know plenty of people (as I'm sure we all do) that can pass a test and still be functionally useless in a particular field.
Not that I'm against exams (and I'll lump in certifications), I just don't think they're the silver bullet many make them out to be at least in the tech industry.
Same way you learn anything, read all about it then use what you read to implement something. When the something you implemented works and solves the problem, you've learned something! Repeat forever with different things (and sometimes with things you haven't done for a while). Each time you learn something new.
Exams are a hacky stand-in for actually using a concept to solve real world problems.
Why do you need external validation of what you know? Is a test going to demonstrate that you know something? Is it going to demonstrate that you know what you need to know, or what the person who designed the test thinks you need to know? And all that aside, what use is the test score? Is that something you'd put on a resume?
Here's my example. I'm currently learning test automation. There are courses for that on Udemy, so I can choose "Test automation with Selenium and Python".
The next step is to google "demo projects for test automation", pick one, and apply that knowledge. If you still need more, pick another.
I suppose the person that assesses my knowledge the most is myself. You know you understand the concept when you can use it and build on it with other learning.
If you're self taught, you can teach yourself the next thing.
You don't have a solid CS background? You can teach yourself that. Go look at, say, the list of MIT courses for a CS degree. You don't have to take the MIT classes. But which of the things do you know, at least to some degree? The ones you don't know, do you feel the lack of any of them? Have they held you back at points in your career so far? Go learn those.
Your work isn't interesting any more? Look around at your company. What work there would be more interesting? What do you need to know to be able to do that work? Go learn that. If things line up just right, you can help out on a project in that area that is short-handed, and get experience on your company's dime. You may have to be able to say "I know a bit about that" first, though, so go learn at least a bit about that.