"How do I know I am good at piano" here's a non-coy, non-sugar coated, direct and honest way I determine if someone is good

1. Fluent in sheet music - this means there is no "visual barrier" to playing any song on piano, any song you see in front of you, you can understand what it should sound like.

2. You can play any song without looking at your hands, and only reading sheet - this shows you've mastered proprioception

3. What you play sounds near identical to the original recording/intention/tempo etc. - this means you have the skill to play a song as intended

So putting it all together

1. I can understand any song you put in front of me just from reading the sheet

2. I can play any song you put infront of me

3. I can play the song how it's originally intended

This is when you are "Good"

notice i didn't say "Great" "Amazing" "Talented", just "Good" which i believe is the first barrier until you can get to later levels where more things come into play that make you an amazinig player like "Speed" "Tempo" "Timing" etc.