I will normally spoiler passive knowledge checks and similar stuff. I want to let you all see this one so you can get a taste of the format I use. Also, its not info that needs to be hidden. Also, I took some liberties with the design and appearance of Casey's skeleton.
[Knowledge Religion]: As you look at the skeleton, you notice a few strange additions. Some joints appear to have metal parts, and many fake ligaments have been rigged with leather bindings. These are things made unnecessary by the dark magics that bind the creature's soul to its animated bones. The most telltale sign that this is no undead, or at least not a typical skeleton is the absence of black onyx in its eye sockets.