When a beam of electrons is incident on a crystal that act a diffraction grating, diffraction pattern is observed.
This experiment shows that electron beam (particle) is behaving as wave and produce diffraction pattern (superposition = wave theory) .Hence electron has wavelength which is comparable to interatomic distance approx 10-10m . Using debroy wavelength formula the wavelength can be calculated if the speed of electron is known.
To achieve this speed the electron has to be accelerated with sufficient potential normally known as accelerating potential.
When electron beam is deflected using parallel plates , we apply acceleration a = qE/m and deflection of electron beam is observed. Apply newton's law F=ma is treating the electron as a particle.
Hence clearly we already know when to consider electron as a particle and when we need to take it as a wave. This is referred to as particle having dual nature.
