How does the pilot assert this condition when planning the flight (to the extent of plausible deniability of deliberate violation) ?
Several ways:
1. Metar from Airport / ask incoming planes
2. Webcams (there are plenty around nowadays)
I meant "VFR on top" in the sense of VMC conditions above the cloud layer.
The common frequency is for blind transmissions (pilot-to-pilot), there is no ATC in uncontrolled airspace. If you start from or ascend into controlled airspace, then of course you talk to the appropriate ATC station.