Clusters of atoms or molecules have been extensively studied by a variety of spectroscopies because of their unusual properties. Experiments with van der Waals clusters of defined sizes are not easily possible because nozzle beam expansions used in their production yield broad size distributions. Moreover, being weakly bound they readily fragment in the commonly used electron impact-ionization mass spectrometer detectors. Here it is shown that light fragile clusters of He, H(2), and D(2) can be selected and identified nondestructively by diffraction from a transmission grating. The method is universally applicable also to heavier species and well suited for spectroscopic studies.