The Monster Manual explains the Skills entry in a monster's statblock on page 8. The section begins with the following sentence:
The Skills entry is reserved for monsters that are proficient in one or more skills.
So if a monster is proficient in a skill, that is reflected in its Skills entry. If a skill isn't listed in the Skills entry, the monster isn't proficient with it.
Like everyone else, monsters can use their ability modifier without proficiency for skills they aren't proficient in. This works out better for monsters then it does for most people, though, because they don't have to deal with point buy - to use your Balor example, it doesn't have an ability modifier less than +2. This would be impossible for a player character using point buy, and incredibly unlikely for a player character rolling their ability scores.