Yeah, my mom bought me some packs last Christmas that looked extremely authentic, but the cards inside were obviously not. I couldn't tell so much as first while I was sitting in the dark, but I did realize that the holo card in each pack was in the back and turned the opposite way of the other cards (face down). Also, the holo cards were usually damaged, which originally lead me to believe the packs were resealed. However, then I go to looking at the cards included in each pack. Most of them were rare, so I basically knew at that point they were fake. I went to a lighter room and evaluated the cards some more. This is when I noticed that the energy symbols were bloated, there were typos on many of them, and the fonts were off. Also, some of the cards had the holo dots in the background but were not reflective. The cards that were holo were too shiny. Most of these cards lacked the accent above the e in Pokemon. Finally, even though I knew for sure these were fake, I gave them the light test. I could see right through them.
In my experience, you are most likely to get these type of cards when shopping at a Dollar store of some kind (where everything is $1). Official Booster packs will usually run you about $3. However, in the past, official Pokemon cards could be found for only $1, but that was years ago. I really miss those days. But there is definately no guarantee that even those more costly are real. In fact, most of those Legends blisters at Wal-Mart (the ones that include 2 or so booster packs + bonus, pre-opened, cards) often include cards that will fail the light test.