We use complementary analysis techniques to determine the structure of nanometric periodic multilayers and particularly their interfaces. We focus on Co-based multilayer which can be used as efficient optical component in the extreme ultraviolet (EUV) range. The samples are characterized using reflectivity measurements in order to determine the thickness and roughness of the various layers, X-ray emission and nuclear magnetic resonance (NMR) spectroscopies to identify the chemical state of the atoms present within the stack and know if they interdiffuse. Results are validated through the use of destructive techniques such as transmission electron microscopy or secondary ion mass spectrometry.