Brakes are one tell, but they can be optioned with the larger brakes.
The only certain way to tell is by the exhaust backboxes. Petrol cars have very visible backboxes that "hang" under the car. Diesel ones have just one resonator that's hidden. It's a bit difficult to tell from 2018+ because they redesigned the bumpers to hide them better, but they're still visible.
Same goes for the Ghibli.