Most reliable way is by the car's chassis number.
On the mk3 this will be:
-stamped on the driver side scuttle area under the wiper blade, lift up bonnet.
-on a metal plaque riveted to the slam panel above the driver side head lamp.
-printed on a sticker in the boot under the carpet or under the tail gate somewhere.
It will read something like WVWZZZ 1H ZVW 123456,
1H is the chassis type
123456 will indicate when it was made as the number car which came off the production line, the main dealer should be able to tell you when.
Other letters, indicate where the car was fab, and some other stuff I can't remember.
