Great article on the impacts of GPS jamming/spoofing here: https://spectrum.ieee.org/aerospace/aviation/faa-files-reveal-a-surprising-threat-to-airline-safety-the-us-militarys-gps-tests
Of course the US Military must prepare for the threat. While our hardware detects outages, we pilots usually fail to detect spoofing events, and if well done our hardware doesn't detect them either. In cruise I'll occasionally verify position by tuning in a nearby VOR and verifying that I'm on the correct radial. However, doing a true position verification using two VORs is often not possible at the altitudes where I most often fly.
In preparation for potential operations in the MON VOR world, having DME would be nice. I've occasionally thought about installing a used DME. What is the go-to model? What about weight/power penalty? Is installing DME worth it as backup nav data to GPS?