Here in Australia, we are about to return to the practice of including ATC frequencies onto the VFR charts and suggesting that VFR aircraft transiting E or G airspace monitor the appropriate ATC frequency when possible. There is concern that this practice imposes a duty of care on ATC to reach out to any aircraft in his airspace if it appears to be in proximity to another, regardless of whether the aircraft is identified, has previously asked for a service, and may or may not be listening.
How do VFR aircraft in USA or Europe (or elsewhere) know what ATC frequency to call for radar advisory service when flying in G or E airspace?