A nondestructive analysis of strained centrosymmetric double-barrier resonant tunnelling diode (DB-RTD) layers is proposed. In the measured (004) rocking curves, the intrinsic barrier-well-barrier sequence results in a beat under the strained layer peak which can be used for layer analysis down to a thickness of nine monolayers. Based on the kinematical theory, analytical relations are deduced which allow a direct correlation of the measured rocking-curve data to barrier and well-layer data. This method is applied to InP-based DB-RTD and is proven to be applicable to final devices including thick but lattice-matched contact layers.