C++ :: How To Check If A Vector Contains Garbage

So i have this code:

if (get_brick_at(Position(row, column)) == NULL)

Where get_brick_at is defined like this:

>Brick& get_brick_at(const Position & p) {
return board[p.get_row()][p.get_column()];
}

Now, of course this does not work since the compiler cannot convert from long int to a Position. So how do i know if the return value of get_brick_at is garbage?

I also have a couple of other situations where i want to return some kind of NULL-like value under certain circumstances.

My question is: How do i handle situations like that?