Just remove the door card and inspect the inside of the door skin for pegs and possibly fasteners... however, if the trim is in line with the side door crash protection beam (which can be located by looking at the door jamb for the spot welds), then it maybe be a bit trickier to find out what‘s underneath the trim. My GTV had original, dealer installed trim which was attached by drilling holes in the door skin. That was pretty standard for that period.