In order to determine whether a phenomenon is a particle or a wave, scientist developed the double slit test, which consists of a barrier that has two slits in it, each of which will allow a particle/wave to pass though. If a stream of particles strikes the barier, it will accumulate in the regions behind the slits. 

When waves converge on two narrow slits, however, constructive and destructive interference results in a series of peaks. The bright areas are places where constructive interference occurs, and the dark areas correspond to destructive interference.