Phantom checkposts are easily set up and lethal to civilians and soldiers alike. With all the technology deployed to keep people safe, solving this problem is a no-brainer.
IFF (Identify Friend-Foe) is a forty-plus year technology geared to giving fighter pilots instant identification of planes in their area of battle. These radio transponders trigger a coded response whenever there is a coded query.
Checkposts could easily use this technology. Transponder codes would be changed daily, and checkposts would be issued a properly coded frequency. Vehicles approaching the checkpost could trigger an IFF request and, if they do not receive the proper response, know that they are driving into a trap.
If we wanted to get just a bit fancier, we could add a small satellite transmission package that would send the location and IFF codes up to US or Iraqi response teams every time an IFF mismatch occurred. That could get drones recording the scene, and air support on the way within seconds of a suspicious checkpoint being queried — maybe even before the shooting starts.