A method of physical testing to determine the natural frequencies of a structure already in service is to apply a short duration force using a hammer or similar instrument and to monitor the time-history response at sensors positioned on the structure. This method is known as a ping or hammer test.

You can simulate a ping test in Mechanical APDL via a transient finite element analysis, then postprocessing via the APDL Math *FFT command. *FFT performs a fast Fourier transformation (FFT) of the acceleration time-history results to identify the resonant responses.