Observers at UC Berkeley stated on March 23 that, with respect to its monitoring in California:
Our equipment would be able to see plutonium particles by observing gamma-ray emissions. We do look for this signature and we do not have any evidence of plutonium at this point.
This is work done at their Department of Nuclear Engineering. Of course, monitoring there is far from the reactors, and presence of different radionuclides depends upon many factors, including weather, and chemical and physical properties of the particles containing the radionuclides. It will be important to follow the plutonium, given how deadly it is.