Self assessment is nearly impossible. Our mind can play tricks on us. Research Dunning-Kruger effect for the science.
Here is a partial solution for music students.
1. Listen to professional recordings of your pieces. Frequently. You should know them as well as you know "happy birthday".
2. Record yourself and listen back to your efforts. Many students find this very difficult. I understand, but, push through the resistance. It is your best self assessment tool that you have.
3. Take lessons. Professional feedback is priceless.
If I can help you, please call me.
You've got to learn your instrument. Then, you practice, practice, practice. And then, when you finally get up there on the bandstand, forget all that and just wail.
I'm a professional pianist and music educator in West Toronto Ontario. I'm also a devoted percussionist and drum teacher.