Well the energy of the photon is simply h.f , so if you can determine the energy of a single photon, you can determine its frequency. One way to determine the energy of a photon; assuming that you can generate one photon at a time, all of the same energy, would be to use the photo-electric effect, with adjustable band gap photo-cathode materials in a PMT, which can detect single photons. Variable bandgap photo-cathodes, can be made over limited ranges from III-V Ternary or quaternary compounds, such as GaAsP or InGaAsP. Smaller bandgap cathodes will emit a photo-electron; higher bandgap ones will not.

Now you didn't say you wanted to know a practical way of doing that, but if you have a need for single photons of known frequency and polarization, making the PMTs should be no problem for you.

Well the energy of the photon is simply $h\cdot\nu$ , so if you can determine the energy of a single photon, you can determine its frequency. One way to determine the energy of a photon; assuming that you can generate one photon at a time, all of the same energy, would be to use the photo-electric effect, with adjustable band gap photo-cathode materials in a PMT, which can detect single photons. Variable bandgap photo-cathodes, can be made over limited ranges from III-V Ternary or quaternary compounds, such as GaAsP or InGaAsP. Smaller bandgap cathodes will emit a photo-electron; higher bandgap ones will not.

Now you didn't say you wanted to know a practical way of doing that, but if you have a need for single photons of known frequency and polarization, making the PMTs should be no problem for you.