Macbook Chargers: Can I take a charger I bought off ebay to the Apple store to see if it's authentic or not?

Will they tell me if it's authentic or not? Is it normal that the length of the cord is a little bit shorter than the authentic one that came with my macbook? Seeing as every single authentic macbook charger on the site ***** anyway (all less than 2 star ratings) I thought I would just buy a cheaper one off ebay that claimed to perform and be authentic.