The method called by a serverRpc is server authorative?

Lets say we have an serverRpc method that calls another method, this other method checks an array and returns a boolean value.

[Rpc(SendTo.Server)]
void CheckArrayServerRpc()
{
   if(CheckArray())
    //do somthing

}

bool CheckArray()
{
   return grid.array.HasEnemy;
}

the checkArray method is server authorative? (consider that this method is only called by an serverRpc).

It is not server authoritative because methods have no authority. It‘s whatever that method performs logically where you can say that it is server authoritative, such as movement or firing a weapon.

As to the method, it simply runs on the server if the server Rpc is the only place where the method is called. The script itself however also runs on clients and a hacked client could still call that method albeit with different effects - it may not have the necessary references or it may try to call a client Rpc which clients can‘t do.