FlightRadar24 has released a new tool allowing you to see where GPS jamming is active
https://www.flightradar24.com/data/gps-jamming
It works by aircraft measuring the strength and quality of the GPS signals they receive when travelling, and then broadcasting this data which is then picked up by FlightRadars network of receivers and fed into the system. Normally, when there is no GPS interference, aircraft will usually get a decent signal. Those areas are marked green on the map. If they are struggling to get a signal, or a very weak one, that could suggest there is GPS jamming in the area. It doesn’t mean there definitely is, because there are a number of different reasons why an aircraft might be struggling to get a signal that don’t include jamming, their equipment could be damaged for example. But if multiple flights in the same area are all reporting weaker signals, that suggests there’s something else going on and FlightRadar will mark that area as red on the map to indicate suspected jamming.