  Experimental determination of entanglement is important not only to
characterize the state and use it in quantum information, but also in
understanding complicated phenomena such as phase transitions. In this paper we
show that in many cases, it is possible to determine entanglement of a two
qubit state, as represented by concurrence, with a few observables, most of
which are local. In particular, rank 1 and rank 2 states need exclusively
measurement of local observables while rank 3 states need measurement of just
one correlation observable in addition to local observables. Only the rank 4
states are shown to require a more detailed tomography. The analysis also sheds
light on the other measure, non separability since it is a lower bound on
concurrence.
