  The determination of the state fidelity and the detection of entanglement are
fundamental problems in quantum information experiments. We investigate how
these goals can be achieved with a minimal effort. We show that the fidelity of
GHZ and W states can be determined with an effort increasing only linearly with
the number of qubits. We also present simple and robust methods for other
states, such as cluster states and states in decoherence-free subspaces.
