All lower spec models got the fat trims regardless of age, but the later GLs got big bumpers from the factory with a different front splitter. They didn't have full arches, but had spats that fit under the bumpers, but still kept the fat trims. I think the late 1.8 Drivers had the option of big bumpers from the factory, but this has not been confirmed.
GTIs from 1990 onwards on a G plate got the big bumpers & skinny trim as standard, but it isn't uncommon to find a late registered early model on a G plate with small bumpers.

The best way to tell if you have an early or late MK2 is to look for the hazard switch, if its on the dash then its an early model, if its on top of the column then its a late spec.