The best way to know for sure is to look where the rear quarters attach to the roof. On vinyl top cars, there is an indentation where the molding attaches on each side. Regular cars, no indentation. The vinyl top cars with the indentation have the chrome molding fitting nearly flush where it meets up by the rear glass moldings. Always a big gap on cars with vinyl top added.
Also, if I remember right, your buck tag would have a code for the vinyl top color.