You can build a cheap and heavily shielded ionization or Geiger system, or a more expensive spectrometry system. In this case, Ruthenium is a condensed solid, and would be ideally detected by an air particle sampling system that continuously concentrates air onto a filter, and measures emitted radiation with a detector capable of resolving very narrow peaks in the spectrum. I would pick up Knoll’s book on detector systems if you are really interested in building such a system.