Land ownership boundaries are a portion of it. Combining that with aerial imagery, street-level imagery, and (in places I have it) local knowledge.

Technically there's a few roads on USFS, BLM, or state land that might be inaccessible, but these are few and far between so that vast majority of time any road or path on public land is going to be public.

With private land, some landowners appear to be fine with letting people cross their land, and some don't. Generally for private land I look for the presence of gates or no trespassing signs to confirm that the land is inaccessible. All the roads/tracks/paths I tag as private I've found something to indicate that the land is likely posted no trespassing or gated. I'm only one person so I'm bound to make the occasional mistake, but that's my methodology that I believe should be pretty thorough. Does that answer your question?