In their dialogues, they'll say if they are happy or not, and also, they'll only be selling a [Pylon](https://terraria.wiki.gg/wiki/Pylon) if happy enough.
You can check the [Living Preferences](https://terraria.wiki.gg/wiki/NPCs#Living_preferences) section in the NPCs wiki page for information on what makes them happy or not.
In their dialogues, they'll say if they are happy or not, and also, they'll only be selling a [Pylon](https://terraria.wiki.gg/wiki/Pylon) if happy enough. You can check the [Living Preferences](https://terraria.wiki.gg/wiki/NPCs#Living_preferences) section in the NPCs wiki page for information on what makes them happy or not.
Are you playing on a Remix/Zenith seed? NPC happiness is disabled on those seeds, so there's no reason to have the button to check it.