The short answer when it comes to differences between the two is "not much". The Prado (or 150-Series in Toyota-speak) has now been on sale in much the same form for more than a decade now. As such, it’s been refreshed and facelift no less than three times, making the ones you’re talking about the Series 4 version.
The thing is, the last major refresh was carried out in late 2017, and since the car continued to dominate its market segment, Toyotas hasn’t really seen the need since to carry out any further refinements or make other changes to the specification of the GXL version.
So, again, the short answer is that there’s very little to choose a 2019 model over a 2018 car. Both were quite wisely updated in the 2017 re-hash, with the GXL gaining pre-collision warning and autonomous braking, active cruise-control, lane-departure warning, LED lighting and a rear differential lock on automatic versions.
The best way to pick which is which is to look at the build-plate in the engine bay as that will give you the definitive answer. Don’t necessarily take the seller’s word for the car’s year of build as the date of first registration is not the same as the build date. Since the trade uses the actual build date when it come to valuing a car, you should too. In fact, that’s the other difference between a 2018 and a 2019 Prado; the price.