I heard it so it must be true

The ticket inspectors have shiny shoes. All of them. Shiny shoes. It's the bribes they take, the money they pocket, folding away their crispy incompleted forms as penitents slip currency into their waterproofs. The briber goes unreported and unblemished, the bribee spends the pocket money on fancy shiny shoes. That's how you can tell the under-cover ones; they're the riders with no bags and shiny shoes.