  We construct a single observable measurement of which mean value on four
copies of an {\it unknown} two-qubit state is sufficient for unambiguous
decision whether the state is separable or entangled. In other words, there
exists a universal collective entanglement witness detecting all two-qubit
entanglement. The test is directly linked to a function which characterizes to
some extent the entanglement quantitatively. This function is an entanglement
monotone under so--called local pure operations and classical communication
(pLOCC) which preserve local dimensions. Moreover it provides tight upper and
lower bounds for negativity and concurrence. Elementary quantum computing
device estimating unknown two-qubit entanglement is designed.
