In linux DPI cannot be detected:

import java.util.Iterator;
import javafx.application.Application;
import javafx.stage.Screen;
import javafx.stage.Stage;

public class JavaFX extends Application {

    public static void main(String[] args) {
        launch(args);
    }

    @Override
    public void start(Stage stage) throws Exception {
        for (Iterator<Screen> it = Screen.getScreens().iterator(); it.hasNext();) {
            Screen s = it.next();
            System.out.println("Screen : " + s);
            System.out.println("DPI : " + s.getDpi());
        }
    }
}

Output:
run:
Screen : javafx.stage.Screen@7150461bbounds:Rectangle2D [minX = 0.0, minY=0.0, maxX=1440.0, maxY=900.0, width=1440.0, height=900.0] visualBounds:Rectangle2D [minX = 0.0, minY=0.0, maxX=1440.0, maxY=900.0, width=1440.0, height=900.0] dpi:0.0
DPI : 0.0