There are many ways to quantify how good of a programmer you are.

- You can compare yourself to other developers (there are plenty of qualities to compare)

- You can review your statistics (commit frequency, average file size, accuracy of time estimates)

- You can compete in programming competitions

- You can evaluate yourself using the Programmer Competency Matrix: https://sijinjoseph.com/programmer-competency-matrix/

Also, "Good" is subjective. Some programmers are "good" in certain contexts, but "not good" in others.