A method for inspecting a sandwich structure may comprise determining a reference frequency, directing first and second laser beams onto the structure, collecting reflected light, processing it using an interferometer, acquiring a time-dependent signal for a predetermined duration greater than a period corresponding to the reference frequency, processing the time-dependent signal to produce a frequency-dependent signal, and comparing characteristics of the processed frequency-dependent signal to the reference frequency. A laser-ultrasonic apparatus configured to inspect sandwich structures may comprise first and second laser beams configured to generate acoustic energy in and illuminate a sandwich structure, respectively, an interferometer configured to receive reflected light and generate a time-dependent signal, detection electronics configured to process the time-dependent signal to produce a time-dependent electrical signal, and one or more processing units configured to convert the time-dependent electrical signal into a frequency-dependent signal and to compare characteristics thereof to characteristics of a reference frequency-dependent signal.