See the PHB, p177.
Arcana. Your Intelligence (Arcana) check measures your ability to recall lore about spells, magic items, eldritch symbols, magical traditions, the planes of existence, and the inhabitants of those planes.
Xanthar on p85 has a guide to this. It's optional, but it's what I would do for identifying a spell.
Identifying A Spell
Sometimes a character wants to identify a spell that someone else is casting or that was already cast. To do so, a character can use their reaction to identify a spell as it's being cast, or they can use an action on their turn to identify a spell by its effect after it is cast.
If the character perceived the casting, the spell's effect, or both, the character can make an Intelligence (Arcana) check with the reaction or action. The DC equals 15 + the spell's level. If the spell is cast as a class spell and the character is a member of that class, the check is made with advantage. For example, if the spellcaster casts a spell as a cleric, another cleric has advantage on the check to identify the spell. Some spells aren't associated with any class when they're cast, such as when a monster uses its Innate Spellcasting trait.
This Intelligence (Arcana) check represents the fact that identifying a spell requires a quick mind and familiarity with the theory and practice of casting. This is true even for a character whose spellcasting ability is Wisdom or Charisma. Being able to cast spells doesn't by itself make you adept at deducing exactly what others are doing when they cast their spells
As such, the DC is 16.
A wolf, at -4 int, would need to roll 20 to recall how to handle sleep, as such. It's possible, but at the edge of plausibility.
In terms of in game stuff, I'd consider several things.
Do you routinely tell your players what spells enemies are casting? Do they have an expectation that everyone will know what spells are being thrown around? If they feel that wolves get meta knowledge that players don't, they may feel the game is unfair and adversarial as the enemy gets info they do not.
Do you give players information for free without knowledge rolls, well above their passive knowledge? If not, they may feel these wolves are unfairly knowledgeable.
Do your enemies routinely show the tactical awareness and skill of highly trained combatants, and not need intellect or wisdom to do so? If so, they may feel the game is adversarial.
In the future if you get into this situation, I recommend rolling their arcana to see if they know, rather than having them simply know spells. In addition, if they question it, turn the question back on them.
"It is weird that they know what sleep does. You wonder who taught them." Because then you can use that as a plot thread, with a werewolf master or some hostile caster using sleep on them.