In terms of wave-particle duality for, let's say a photon; how would the frequency practically manifest/demonstrate itself? Like, i understand that the frequency is related to the energy a particle has, but frequency in my mind suggests oscillation about a point. Is the photon physically oscillating through space as it travels? I wouldnt imagine so. Which periodic occurrence is referred to when one talks about the frequency of a particle?
No the photon is not oscillating through space. It is an elementary particle of the standard model which is the quantum mechanical description of most of our experimental knowledge on elementary particle to date. Elementary particles are point particles.
The classical wave is built up from an enormous number of photons and as physics theories have to be consistent when the parameters and variables change from microscopic to macroscopic the frequency entering the E=h*nu for the individual photon is the frequency built up in a coherent classical beam emerging from a large ensemble of photons.
Conversely, when one starts with a classical coherent beam of frequency nu, as discussed in the other answer, and one goes to the microscopic level of individual quanta that compose the beam, the photons, the frequency identifies the quantum of energy the photon carries.
For example, a large number of excited atoms at the same energy level ( lasers for example) de exciting to photons will have the energy h*nu that builds up the classical wave.
For what is a particle in the quantum mechanical microcosm see my answer here.
The easiest way to see frequencies is in interference. Imagine you have waves coming towards a wall. Imagine too that the frequency of the waves is way higher that what you can see. You cannot directly observe the waves, but you will see that the wall is wet a few centimetres over the surface. Now, instead of one wave, you have two coming from different points. In this case, you will see that your wall has wet areas and dry areas.
The distance between the dry and wet areas depends on the size of the waves, and assuming they go at a uniform speed, the size (or wavelength) and frequency are inverses.
How do you measure this experimentally? Pretty much this very same way, take light beams and make them interfere, the size of the features will be proportional to the size of the light, or inversely proportional to the frequency.