To be honest, I like some of the well educated and washed middle-class ones. Maybe they should do nightclub style 'face control' at airports. If you look like Uncle Ganbei or some other farmer, then they're not going to let you board that plane.

Indicators of passengers being dangerous could include a tendency to carry a man purse, chain smoke Chungwa cigarettes, buy the most expensive tasteless blingy mobile phone possible rather than the much cheaper yet most technologically advanced one, half of their teeth falling out, those cheap fake leather shoes/Hermes belt, polo shirts tied up to cover up the nipples, yet expose a baijiu belly and that smell that most likely comes from not showering for three days...

Feel free to add to that non-exhaustive list.