Class skills are typically whatever the monster entry has ranks in, once you subtract and account for racial mods.
Basically, compare the skill list to the racial skill mods listed in the text, if any. Any skill with a racial mod, subtract the mod and see if the listed skill total is different from just the base ability modifier for that skill. Most ones aren't. Anything left over is a class skill.
Most All Almost all animals just have spot and listen, for example. (The medium shark, for instance, has spot, listen, and swim. But swim is a +9, with +8 of that being from the swim speed, and the last +1 from strength mod. Thus, not on the list.)