If you just consider consecutive samples then, for most kinds of PRNGs the answer is that you can't distinguish them from true RNGs.

If you consider large input samples, then, in principle (if you don't take complexity issues into account), you can always distinguish a PRNG from true randomness:

For each PRNG $r$ and for each seed length $L$ there exist a sample sequence length $k$ such that $\forall x,\  f([r(seed, x),\ r(seed, x+1),\ \dots,\  r(seed, x+k-1)])\ =\ 1$ with certainty, and $f(\dots)\ =\ 0$ for any other sequence,
where $r(seed, n)$ is the n-th sample generated by the PRNG initialized with $seed$.