Detecting keyframe on current time

You'd use a combination of the nearestKeyIndex(time) and keyTime(index) property methods.

For example:

var activeItem = app.project.activeItem

if (activeItem != null && activeItem instanceof CompItem) {

  var prop = activeItem.selectedProperties[0];

  if (prop != null && prop.numKeys > 0) {

    alert("current time = " + activeItem.time + " nearest key time = " + prop.keyTime(prop.nearestKeyIndex(activeItem.time)));

    if (prop.keyTime(prop.nearestKeyIndex(activeItem.time)) == activeItem.time) {

      alert("There's a keyframe at this time");

    }

  }

}