How to look if PlayerPref exists at the beginning of the game?

Well if you check the documentation you will notice that there is a HasKey method w$$anonymous$$ch tells you if a certain key currently exists or not. However in almost all cases you don't need to check t$$anonymous$$s manually. All the Get methods (like GetInt) have an optional second parameter where you can specify the default value w$$anonymous$$ch is returned in case the value does not exist yet. So inside Start you can simply do

 const string m_SecretUnlockedKey = "secretUnlocked";
 bool isUnlocked;
 
 void Start()
 {
     isUnlocked = PlayerPrefs.GetInt(m_SecretUnlockedKey, 0) != 0;
 }
 
 void UnlockSecret()
 {
     isUnlocked = true;
     PlayerPrefs.SetInt(m_SecretUnlockedKey, 1);
 }

Assuming your score is an integer, you'd set it with PlayerPrefs.SetInt().

In answering the question in your subject, you check whether it has a key with PlayerPrefs.HasKey().

Thus, for example, in a script on your player:

     private static int $$anonymous$$ghScore=0;
     private void SetStandardAnimations(GameObject o) {
         if(PlayerPrefs.HasKey("HighScore"))$$anonymous$$ghScore=PlayerPrefs.GetInt("HighScore");
     }
     private void OnDestroy(){
         PlayerPrefs.SetInt("HighScore",$$anonymous$$ghScore);
     }