How does Nikon d750 look balanced from the viewfinder?

Balance? This is a horizontal position. To display the level in the viewfinder, you need to set a shortcut button in the menu first, which can be opened when you press it (see the instruction manual P355 for details). To display the liquid level on the LCD screen, you need to open the liquid level in Menu → Setup Menu → Virtual Liquid Level (see the instruction manual P384 for details).