First thing to be aware of is the
Dunning-Kruger Effect.
Next is to measure the responses of people who read your work. If you post a story on your blog and your regular readers, unprompted, post links to it, you're doing well.
There's really no other way I'd measure competence. When you publish a novel, its success will depend on the word of mouth of strangers; if no one seems interested in spreading the word, there's more work to do.
This also does an end run around issues of "good" and "bad" books. While discussions of whether a kind of book is good or not is worthwhile (and fun!) it's actually a millstone around the neck of a writer trying to break in. There will always be people who dislike your books (even mine, if such a thing can be imagined). But are there readers who want to share it?
The struggle to improve your work is another matter. Every writer needs different help, but there are a couple of tricks to try.
First, as Uncle Jim suggests, retype the first chapter of a favorite novel. Especially a novel similar to the one you're writing. It doesn't take very long, but it will help you with sentence structure, voice, and all the good stuff.
Then, hold up the first page of your work and the first of the retyped one. How do they compare? Are they just as interesting? Do things happen in an original way?
Then go through the first three pages of your manuscript. Highlight all the verbs. Are they interesting? If not, don't try to change the words, look at the story you're telling.
Then read a book similar to yours that you'd like to emulate, and outline it. How long until the main conflict is established? How much page-space on exposition? On outright conflict? How many plot twists, and how often do they come?
And so on. Another useful thing is to read others' descriptions of what they like and dislike.
At least, those are the things that helped me. Each writer has different strengths and weaknesses, so each would need different medicine.
Good luck.
