Neutrinos
If their braking propulsion is showering inner solar system with narrow beam of highly energetic neutrinos from exotic nuclear reactions, they are detectable with existing neutrino detectors and it is possible to determine the direction.
Theoretical models of such propulsion already exists and it will beexist. It is possible to check the source with existing x-ray and gamma-ray telescopes/spectrometers whether the spectrum conforms to these models. Compared to visible spectrum and radio, the x-ray and gamma-ray space background is quieter and italiens would stand out much more. This way it can be determined that it is artificial process and measured their speed by doppler shift.