How can I tell what a beast is proficient in for the Beast Master?

Question

With the following statement in D&D 5e's Player Handbook > Chapter 3 > Ranger > Ranger Archetypes > Beast Master

Add your proficiency bonus to the beast’s AC, attack rolls, and damage rolls, as well as to any saving throws and skills it is proficient in.

How do I know what saving throws and skills the beast is proficient in? For example the Hawk's stat block says

Skills Perception +4

Does that mean that the Hawk is proficient in perception and I should only add my ranger's perception to that skill? If so why is saving throws even written? There is no beast that could possible have a proficiency in saving throws?

Answer 1

Yes, the Skills and saving throws mentioned in the Stat Block are the skills and saving throws which the beast is proficient at.

Skills

As your answer mentioned, the Skills subsection at (DMG 8) states that

The Skills entry is reserved for monsters that are proficient in one or more skills. For example, a monster that is very perceptive and stealthy might have bonuses to Wisdom (Perception) and Dexterity (Stealth) checks.

A skill bonus is the sum of a monster’s relevant ability modifier and its proficiency bonus, which is determined by the monster’s challenge rating (as shown in the Proficiency Bonus by Challenge Rating table). Other modifiers might apply. For instance, a monster might have a larger-than-expected bonus (usually double its proficiency bonus) to account for its heightened expertise.

Saving Throws

The Saving Throws entry is reserved for creatures that are adept at resisting certain kinds of effects. For example, a creature that isn't easily charmed or frightened might gain a bonus on its Wisdom saving throws. Most creatures don't have special saving throw bonuses, in which case this section is absent.

A saving throw bonus is the sum of a monster's relevant ability modifier and its proficiency bonus, which is determined by the monster's challenge rating (as shown in the Proficiency Bonus by Challenge Rating table).

Oddly, though the BM section you mentioned states

Add your proficiency bonus to the beast’s AC, attack rolls, and damage rolls, as well as to any saving throws and skills it is proficient in.

There seems to be no beast proficient at any saving throw, so I suppose this part is intended for homebrew content, future releases or just a mistake.

Answer 2

I found the answer in the Monster Manual > Intrduction > Statistics > Skills

The Skills entry is reserved for monsters that are proficient in one or more skills. For example, a monster that is very perceptive and stealthy might have bonuses to Wisdom (Perception) and Dexterity (Stealth) checks.

A skill bonus is the sum of a monster’s relevant ability modifier and its proficiency bonus, which is determined by the monster’s challenge rating (as shown in the Proficiency Bonus by Challenge Rating table). Other modifiers might apply. For instance, a monster might have a larger-than-expected bonus (usually double its proficiency bonus) to account for its heightened expertise.