Event Listner

When you place something you trigger four (relevant to your question) events: beforeImport, afterImport, beforePlace, afterPlace.

When you consider the Place Gun (i.e. the loaded cursor you get when you manually place a file) the meanings are these:

• beforeImport: ID ist about to load the place gun
• afterImport: ID has loaded the place gun
• beforePlace: the user has clicked with a loaded place gun
• afterPlace: ID has placed the file at the location of the click

Problem: Unless you create links for text files the beforeImport/afterImport event does not know what it is that its importing. (Someone please correct me if this is wrong).

The only solution I can currently think of would be to hook into the call of the menu item "Place" with MenuAction.afterInvoke and handle the file-selection etc yourself.

FWIW, here is a snippet that installs most of InDesign's event listeners to help you see, when what happens.

//EventListenersOn.jsx
//An InDesign CS5 JavaScript
//
//Installs event listeners for many supported events; displays a
//message when each event occurs.
#targetengine "session"
main();
function main(){
    mySetup();
    mySnippet();
    myTeardown();
}
function mySetup(){
}
function mySnippet(){
     var myEventNames = [
          "beforeQuit", "afterQuit",
          "beforeNew", "afterNew",
          "beforeOpen", "afterOpen",
          "beforeClose", "afterClose",
          "beforeSave", "afterSave",
          "beforeSaveAs", "afterSaveAs",
          "beforeSaveACopy", "afterSaveACopy",
          "beforeRevert", "afterRevert",
          "beforePrint", "afterPrint",
          "beforeExport", "afterExport",
          "beforeImport", "afterImport",
          
          "afterContextChanged",
          "afterDelete", "beforeDelete", 
          "beforeDisplay",
          "afterEmbed", "beforeEmbed",
          "afterInvoke", "beforeInvoke",
          "afterLinksChanged", 
          "afterMove", "beforeMove",
          "afterPlace", "beforePlace",
     /*     "afterSelectionAttributeChanged", 
          "afterSelectionChanged",     */
          "afterUnembed", "beforeUnembed",
          "afterUpdate", "beforeUpdate"
     ] ;
     for (var myCounter = 0; myCounter < myEventNames.length; myCounter ++){
          app.activeDocument.addEventListener(myEventNames[myCounter], myEventInfo);
     }
}
function myTeardown(){
}
function myEventInfo(myEvent){
     try{
          var myString = "Application Event: " +myEvent.eventType;
          myString += "\r\rTarget: " + myEvent.target.constructor.name + ", " +myEvent.target.name;
          myString += "\rCurrent: " +myEvent.currentTarget.constructor.name + ", " + myEvent.currentTarget.name;
          myString += "\r\rPhase: " + myGetPhaseName(myEvent.eventPhase );
          myString += "\rBubbles: " + myEvent.bubbles;
          myString += "\r\rCancelable: " +myEvent.cancelable;
          myString += "\rStopped: " +myEvent.propagationStopped;
          myString += "\rCanceled: " +myEvent.defaultPrevented;
          myString += "\r\rTime: " +myEvent.timeStamp;
          alert(myString);
          function myGetPhaseName(myPhase){
               switch(myPhase){
                    case EventPhases.atTarget:
                         myPhaseName = "At Target";
                         break;
                    case EventPhases.bubblingPhase:
                         myPhaseName = "Bubbling";
                         break;
                    case EventPhases.done:
                         myPhaseName = "Done";
                         break;
                    case EventPhases.notDispatching:
                         myPhaseName = "Not Dispatching";
                         break;
                    default:
                         myPhaseName = "no phase defined";
               }
               return myPhaseName;
          }
     }
     catch(myError){
          alert("Error getting event properties.");
     }
}

Is this what you are looking for?

InDesign ExtendScript API (8.0)