When an idea sticks with you for long and persists till you put it on paper, then you know it’s a good idea. Maybe not writing everything down is a helpful process itself, because time and memory will filter out the bad ones. I remember writing my first novel and I was so overwhelmed with brainstorming that I would lay to bed and couldn’t sleep since I was bombarded with ideas all night long. I have never experienced something like that since then. And I had this great idea about my character. I didn’t write it down and then I forgot it, but when I engaged with the process of writing my character, the idea popped out of nowhere and I remembered and then I was 100% sure that idea was pure gold.