How can I know if I need to delete something in C++?

Imagine the following class:

class MyString {
public:
    const char* str;
    std::size_t str_len;

    MyString(const char* str, std::size_t str_len)
        : str { str }
        , str_len { str_len }
    {}
}

I'm a bit confused about implementing a destructor for MyString. My first thought was that it would look like this:

~MyString() {
    delete [] str;
}

But how can I delete str if I can't be sure that it was allocated? For example, I could create an instance of MyString like this:

const char* c_string = "Hello, World!";
MyString my_string(c_string, 13);

in which case I shouldn't delete str because it was not declared on the heap, but if I created an instance of MyString like this:

char* char_array = new char[13]{'H','e','l','l','o',',',' ','W','o','r','l','d','!'};
MyString my_string(char_array, 13);

not deleting str would cause a memory leak (I assume) because it would be declared on the heap. But if I created an instance of MyString like this:

char* char_array = new char[13]{'H','e','l','l','o',',',' ','W','o','r','l','d','!'};
MyString my_string(char_array + 3, 10);

I shouldn't delete str because although it's on the heap, it hasn't been allocated; it just points to part of something else that's been allocated.

So how can I be sure that I'm not deleting something that I shouldn't be deleting or failing to delete something that needs to be deleted? Would the answer be different if MyString used char*s instead of const char*s? What if I used MyString my_string = new MyString...?