Here are the key details collectors look for:
The Mechanism: Holographic foils are heavier than un-foiled cards. While "light" packs usually contain 20.2g to 20.7g of cardboard, heavy packs have that extra 0.5g to 1g needed to push the total weight to ~21.0g+, virtually guaranteeing a holographic pull inside. [1, 2, 3]
Set & Artwork: The Fossil set (the third main expansion of the TCG) is recognizable by prehistoric Pokémon art. Popular pack arts feature Lapras, Aerodactyl, or Zapdos. [1, 2, 3]