How can you tell if your vw is a TDI or GTI?

Open the fuel filler door. If it says "diesel fuel only" inside, it is a TDI; if it says "unleaded fuel only" it cannot be a TDI because TDI is a diesel engine. As to telling if it's a GTI...all GTIs I know of have thin red accent stripes, but if you got yours used it's possible the previous owner removed all signs of your car's GTIness.