In (supersonic) molecular beams  temperatures are very low (usually less than 5 K). At these low temperatures Van der Waals (vdW) molecules are stable and can be investigated by microwave, far-infrared spectroscopy and other modes of spectroscopy. 
Also in cold equilibrium gases vdW molecules are formed, albeit in small, temperature dependent concentrations. Rotational and vibrational transitions in vdW molecules have been observed in gases, mainly by UV and IR spectroscopy.
Van der Waals molecules are usually very non-rigid and different versions are separated by low energy barriers, so that tunneling splittings, observable in  far-infrared spectra, are relatively large. 
Thus, in the far-infrared one may observe intermolecular vibrations, rotations, and tunneling motions of Van der Waals molecules. 
The VRT spectroscopic study of Van der Waals molecules is one of the most direct routes to the understanding of intermolecular forces.