Dagenham - I think the 'tap-test' is more about identifying any voids in the structure - areas where the fibres and resin are not homogenous and weakness could be present.
The Lynx Composite Main Rotor Blades (CMRBs) were checked regularly in the hangar using a 2 pence piece which was tapped along the blade whilst listening for any change in tone.
I remember hearing that the a tap-test procedure for Sea King blades uses a halfcrown. Whilst in-service non-destructive testing for current platforms includes far more advanced techniques (such as ultrasonics), Westland inspectors will still carry a halfcrown in their kit for the Sea King...