  We propose a scheme based on using the singlet ground state of an electron
spin pair in a double quantum dot nanostructure as a suitable set-up for
detecting entanglement between electron spins via the measurement of an optimal
entanglement witness. Using time-dependent gate voltages and magnetic fields
the entangled spins are separated and coherently rotated in the quantum dots
and subsequently detected at spin-polarized quantum point contacts. We analyze
the coherent time evolution of the entangled pair and show that by counting
coincidences in the four exits an entanglement test can be done. This set-up is
close to present-day experimental possibilities and can be used to produce
pairs of entangled electrons ``on demand''.
