What is the purpose of casting detect spell in QFG3 at pool of peace? It shows E sign and that's all.

To show you who made the Pool of Peace :)

It's for world-building and for flavor. It serves no practical purpose, but not everything has to.

In fact, there's many places in the series where you can cast Detect Magic to learn something about the world around you with no other purpose, especially in the fourth game. You can do this in the fifth game as well, the only issue is, you have to cast Detect Magic by clicking on it in your shortcut bar and clicking on what you want to cast Detect on - simply pressing the keyboard key will result in the hero casting it on *himself*.