  Certifying entanglement of a multipartite state is generally considered as a
demanding task. Since an $N$ qubit state is parametrized by $4^{N}-1$ real
numbers, one might naively expect that the measurement effort of generic
entanglement detection also scales exponentially with $N$. Here, we introduce a
general scheme to construct efficient witnesses requiring a constant number of
measurements independent of the number of qubits for states like, e.g.,
Greenberger-Horne-Zeilinger states, cluster states and Dicke states. For four
qubits, we apply this novel method to experimental realizations of the
aforementioned states and prove genuine four-partite entanglement with two
measurement settings only.
