There should be a sticker inside the driver's door jamb. This'll provide details such as the VIN and build date of the car. Lots of new cars have spent extended periods in storage over the last 18 months. It shouldn't be an issue, as the dealer will carry out a pre-delivery inspection to make sure the car is as new.

One thing to bear in mind is that a model's standard specification can change regularly. If you're buying a car that was built a year ago, it might not necessarily be identical to a brand new model. Double-check it has everything on it you want.