Pack-searching (1): Since the value of what's inside a pack can vary greatly from one to another, unethical people will go to great lengths to find the packs containing the best cards, especially now that there are many cards that can be pulled from packs that are worth hundreds or thousands of dollars. Usually, the search is as simple as comparing how a pack feels. It may be more invasive. Older wax packs are very susceptible to this because of their simple packaging. A wax-sealed pack can be pried open, the cards searched and cherry-picked, and replaced, possibly with a common taking the place of a star. The wrapper can then be refolded and the wax then melted and resealed. A skilled perpetrator can do this and get away with it without leaving any evidence. Other pack-searching techniques include weighing packs, using calipers to determine thickness, passing them through metal detectors, using fiber-optics to peek into the packs, or peering through the wrapper to see the name on the front and/or back card. Some jerks might just pull open the pack a little bit and take a peek. Usually this behavior is encountered at mass-retailers where no one is watching like K-Mart.
Pack-searching (2): Sometimes the manufacturer makes a mistake. Sometimes there are noticeable patterns in card collation. With the old wax packs, you could see who was on the top and bottom of the pack. If the card makers were lazy, it might be true that if you had a pack with Tim Foli on the front, there would be a Mike Schmidt six cards down. Sometimes it's the pack location that's a giveaway. 1992 Donruss, for example, seeded all the packs containing Diamond Kings inserts on the same level. A box contained four stacks of nine packs. If you opened packs from one stack down to, say, the fifth pack where you pulled a Diamond King, you could be sure that the fifth pack in the other three stacks also contained a Diamond King. 1999 Topps Chrome Football supposedly also had all the expensive, short-printed rookies on the same level. At $6 per pack, you had best find a box that had been freshly opened or well-shuffled by the dealer. In 1992, Upper Deck included a special pack in each box that contained cards from a separate set called Quarterback Club. This pack looked almost identical to the regular packs, but the fin seal on the back of the foil Pack was distinctly different from the rest. Once you knew what to look for, that pack was easy pickings. In other cases, though I don't know for sure, I've heard you could figure out a pattern and find the key cards within a case by deciphering packing codes and box markings.