We don't. Pangram is HN's astrology. It has 99.99% accuracy (according to Pangram.)
"Detect AI-generated content with 99.98% accuracy." -pangram themselves
But there isn't and won't be such a tool. As a fallback people defend Pangram etc. Posting Pangram scores in comments is fundamentally no difference than posting ChatGPT responses in comments, and it's quite hilarious that the former is normalized on HN.