Most publishers of collectible trading card games include “premium foil cards” in their print runs in order to “increase collectibility” and profits. These special rares are supposedly indistinguishable from the back, but the face of the card has a metallic sheen from a thin sheet of mylar. While this augments the basic design, what makes premium foil cards so fascinating to the scrubs is the highly increased price of such cards. Most younger players regard collecting premium foil cards as essential and will trade nearly any other card for it. (This can be exploited in sealed deck game formats that allow trading among players.)
For Magic: the Gathering, there is said to be 1 premium foil card for every 100 cards. This is approximately 5.4 premium foil cards in each box of 36 booster packs (15 cards each). Now pay attention here, it seems that the foil card is heavier than the non-foil card by about 0.1 grams. If you have access to an electronic gram scale and lots of unopened booster packs, you may sort them by weight and thereby determine which contain premium foil cards.