How do you understand that a task is done qualitatively?

A task is considered to be done with quality if it meets several criteria:

• Compliance with requirements: all functionality described in the technical specifications or user story is implemented.
• Correctness: the code runs without errors, covers all intended scenarios, including boundary cases.
• Testability: there are automated tests that verify the main and critical paths.
• Readability and maintainability: the code is written clearly, adhering to standards and style, which facilitates future support.
• Performance: the solution efficiently uses resources and meets response time requirements.
• Documentation: if necessary, comments and documentation are provided to explain complex points.

For example, in Go, I would check that the function is covered by unit tests, does not contain data races, and that the code is structured in a way that makes it easy to extend and maintain.