Given a path on some navigation program such as Google Maps or Waze in the United States, how can I know ahead of time (= before driving) whether some parts of the path are not covered by cellular network (i.e., not possible to make phone calls or access Internet)?
Your best bet is to look up the coverage map for your carrier. The major carriers (AT&T, Verizon, Sprint/T-Mobile) all have maps, and you can look at your planned route and see what the coverage is supposed to be along the way. I'm not aware of any automated way to integrate this process with a navigation program.
The reality though is that these maps aren't all that accurate and often overstate the actual coverage. I'd use them to get a general idea of what to expect—if the area you're in shows no coverage, then I'd expect no coverage—but not as any real assurance that you'll have coverage everywhere indicated on the maps.