How does Katniss know a hovercraft is about to approach the arena during games?

Katniss doesn't know a hovercraft is about to approach the arena during the Games. There is no mention of a hovercraft arriving during any of the Games.

It's likely you're thinking of a different event:

* The hovercraft that brings the tributes into the arena: This is the initial arrival of the tributes to the Games. It's a dramatic scene where the tributes are taken from the Capitol to the arena.

* The hovercrafts used for Capitol viewing: These are used by Capitol citizens to get a close-up view of the Games. They aren't used to approach the arena during the Games themselves.

Please clarify what scene you are thinking of, and I can give you a more specific answer.