  We show that the quantum Fisher information provides a sufficient condition
to recognize multi-particle entanglement in a $N$ qubit state. The same
criterion gives a necessary and sufficient condition for sub shot-noise phase
sensitivity in the estimation of a collective rotation angle $\theta$. The
analysis therefore singles out the class of entangled states which are {\it
useful} to overcome classical phase sensitivity in metrology and sensors. We
finally study the creation of useful entangled states by the non-linear
dynamical evolution of two decoupled Bose-Einstein condensates or trapped ions.
