Anesthesiologists take an intuitive eyeball approach to anticipating difficult intubations, but many formal scores and assessment tools of varying sensitivity and specificity exist, like the Mallampati or Wilson scores. This full text JAMA review looks at some of these in predicting difficult intubation.
The best predictors were an inability to bite the upper lip with the lower incisors, a short hyomental distance, retrognathia, or a combination of findings based on the Wilson score.
The inability to bite the upper lip with the lower teeth was the best predictor.
A good full text review at this link