1. A method of investigating performance of a mechanical system operating under in-situ operating conditions, wherein the mechanical system is responsive to an operating input loading so as to have an operating response to the operating input loading, the method comprising: collecting, via a first sensor, input load data corresponding the operating input loading, wherein the first sensor is not an operational part of the mechanical system; collecting, via a second sensor, response data representing the operating response, wherein the second sensor is not an operational part of the mechanical system; causing a machine to generate an in-situ representation of the mechanical system as a function of the input load data and the operating response data; obtaining a target-response representation of the mechanical system; and comparing the target-response representation and the in-situ representation with one another so as to determine the performance of the mechanical system.