  We present observable upper bounds of squared concurrence, which are the dual
inequalities of the observable lower bounds introduced in [F. Mintert and A.
Buchleitner, Phys. Rev. Lett. 98, 140505 (2007)] and [L. Aolita, A. Buchleitner
and F. Mintert, Phys. Rev. A 78, 022308 (2008)]. These bounds can be used to
estimate entanglement for arbitrary experimental unknown finite-dimensional
states by few experimental measurements on a twofold copy $\rho\otimes\rho$ of
the mixed states. Furthermore, the degree of mixing for a mixed state and some
properties of the linear entropy also have certain relations with its upper and
lower bounds of squared concurrence.
