So when do you know ?
I’ve closed my eyes and let a flame spell it out for me by terms of sigil or name.
Familiarity with “scanning an image” into your mind with crisp recall helps to develop this.
A lot of sigils are in my head, a few memorized.
Flashing your eyes at the sigil at random timing helps to show it to your closed eyes.
You can find a way. Knowing correspondences by heart may help you.