How do I know at Fortnite on PC whether the opponent is playing controller, the professionals notice it immediately?

PS: Is it really worth playing on the controller on the PC, or to start or rather M uK?!

So: If you notice that the opponent has help aiming, then he is playing on a controller. In addition, players on controlers can usually not move as smoothly as mouse keyboard people. You can learn to build on controllers better and faster at the beginning and you have goal support. If you can build and aim well, stick with the mouse and keyboard. Hope I helped.