  Measurement of an observable on a quantum system involves a probabilistic
collapse of the quantum state and a corresponding measurement outcome. L\"uders
and von Neumann state update rules attempt to describe the above
phenomenological observations. These rules are identical for a nondegenerate
observable, but differ for a degenerate observable. While L\"uders rule
preserves superpositions within a degenerate subspace under a measurement of
the corresponding degenerate observable, the von Neumann rule does not.
Recently Hegerfeldt and Mayato [Phys. Rev. A, 85, 032116 (2012)] had formulated
a protocol to discriminate between the two types of measuring devices. Here we
have reformulated this protocol for quantum registers comprising of system and
ancilla qubits. We then experimentally investigated this protocol using nulear
spin systems with the help of NMR techniques, and found that L\"uders rule is
favoured.
