A block cipher in counter mode creates a key stream by:
$$z_i = E_K(IV || i)$$
Where $IV$ is some chosen initialisation vector (with some length $l$ less that $n$ bits) and $i$ is the binary representation of the number $i$ in $n-l$ bits. The keystream is then $z_1 || z_2 || z_3 || \cdot \cdot \cdot$
How can we distinguish between such a key stream and a truly random bit sequence?