Maybe a few bootlegs were released on cdr then. . . .
The easiest way to tell cdr from cd is "tint." Most common these days is a light green tint. There are also blue, purple, black. . . Some are hardly tinted at all though, that's become quite common on purchased cdrs.
Plus they just look different, they don't look as if they're uniformly a plated finish, They often to me seem to look as if they're covered with a host of concentric circles barely visible.