How to spot a fake macbook pro charger?

I have recently had trouble with a local store in my student area selling what seems to be legitimate macbook pro chargers, however, after purchasing one and it breaking I got a replacement, the second time it broke again within three weeks. The guy in the shop then refused to give me a refund or replacement, telling me that it was a problem with my laptop. I then however visited a proper apple store they confirmed it was a fault with the charger and not with my laptop.

The apple store guy could not be sure but told me he believed the product to be fake, I was wondering if there is any way to legitimately find out if the product is a fake? Thanks!