  We propose to detect quantum entanglement by a condition of local
measurments. We find that this condition can detect efficiently the pure
entangled states for both discrete and continuous variable systems. It does not
depend on interference of decoherence from noise and detection loss in some
systems, which allows a loophole-free test in real experiments. In particular,
it is a necessary condition for the violation of some generalized Bell
inequalities.
