ETA: a few more thoughts.
I know my needs / desires before I shop. What's missing? What do I feel I need (and don't have) when I'm trying to put together outfits? How do I want to look and what pieces are necessary to achieve that look?
At the same time, I stay open to the wild cards. (They often turn out to be game changers.)
As others have said, if I can immediately imagine a bunch of outfits with this item and see myself wearing it in different contexts, that is a really good sign.
A few of my "absolute right, I know I'm buying (or keeping) this" purchases over the years. As I went through my Finds I realized almost ALL of them would qualify. But these are pieces I felt instantaneously YES about. Some are essentials or completers. Some are statements. Some I may have asked the forum about, but even if I did come for K/R advice I was probably really asking everyone to love my beautiful thing with me.