The topic of code quality comes up often enough that there is a wiki Q&A devoted to it:
There are currently 103 questions linked to or marked as a duplicate of that question.
The question is very broad, and a poll asking for ideas. There are no constraints on the problem, and there is no single correct answer.
How would one know if the code he has created is easily maintainable and readable? Of course in your point of view (the one who actually wrote the code) your code is readable and maintainable, but we should be true to ourselves here.
How would we know if we've written pretty messy and unmaintainable code? Are there any constructs or guidelines to know if we have developed a messy piece of software?
The answers are equally as broad to the point of being useless.
Your peer tells you after reviewing the code.
Sometimes, the best way to know, is to come back to code you wrote six months ago and try and understand what it was written to do.
It is: maintainable if you can maintain it.
If your code follows the principles of SOLID and DRY and has a good set of unit tests around it, it is probably maintainable.
If you can understand it after 6 months, it's not bad.
Yes, these are all correct answers. None of them point the question's author in the wrong direction. But there is no success criteria. There is no real problem to be solved. Why ask the question? Does your team have a specific quality issue to solve? No, this is a poll, and the answers are vague, abstract, and plain old common sense to anyone in the industry.
This is compounded by the fact that there are over a hundred questions marked as a duplicate of this one. A question is a duplicate when the duplicate target's answers can also answer the new question. With the quality of the answers being so poor they cannot answer the duplicate target question, how could they answer the question being marked as a duplicate? They cannot.
What should we do to this question? Is it worth improving its quality, or should it be locked or deleted? What should be done with the linked questions marked as a duplicate of this question?