Nibbie's got the right idea. A notch/convertible spoiler will have a nearly straight base to it, whereas a GT/LX (hatch) version of any spoiler will have a noticably steeper angle to it. If you can lay it flat on a table or countertop or something, see what kind of angle it sticks up at. The steep angle of a hatchback wing should be pretty obvious once you lay it on something flat.
If you're really good at eyeballing things, though, you can just kind of hold it up and look at it straight from the side, level the wing, itself, out to where it should be if it were mounted, and THEN look at the angle of the base. (Same principle as above, just checking the base angle instead of the wing.)
I believe you can check some pics posted on aftermarket body piece manufacturer sites (Cervini's, ABC Exclusive, etc.) and they'll sometimes have close-up examples of a hatch and a notch/'vert wing so you can see the difference in angle.
i hope i made sence.
