If they want to enforce it -- they will bring in a few ground radars in "hot"-zones -- and if you are close enough to an airport - these folks know their stuff.
From their perch, they usually have a better view of the surroundings than most realize. They may not know where you launched from -- but they will have a GREAT idea if it becomes a problem -- where to tell these enforcement guys to point their radar.
As suggested by Ansia: They have upped their game in being able to track these things over the last few years, as demonstrated at Superbowl. As much as we would like to think these people have set on their butt's the last few years and just kept mum -- nah -- they've been watching. Like Santa -- but the gifts they bring -- do not bring joy and make a lump of coal look like candy.
... that is until they find something else to sink their enforcement teeth into...
Edit: Another thing people forget -- the US may not be like other countries under "strict" surveillance -- with a camera on every street corner... But our government has flown these as part of "our" national security for years. One way or another "Big Brother" is watching. Just because you do not see the portable radar all of a sudden in your area -- or the camera on their drone pointed at you -- doesn't mean they can't see you.
Military drones - the new air force
View attachment 3865
FBI director says surveillance drones used in US
View attachment 3861