Factory vs Aftermarket parts

[XFive3.0]

How can u tell the difference between parts purchased at a BMW dealer versus parts purchased at a local parts dealer like Napa, AC Delco etc.?

In the past I've always bought pads and rotors at the BMW dealer and had a family friend/mechanic put them on for me in hopes of saving a few bucks on labor cost.

Well last week I brought my baby in his shop to replace the O2 Sensor. While it was in there I decided to have him do the rear breaks. He assured me that he only puts parts that come right from the dealer. But just to be sure that I'm not getting ripped off I just want to be sure for my own peace of mind.

I appreciate any input on this.

2001 X5 3.0 - Alpine White 106K mi.