Have a look at fl.addEventListener()
in the JSFL reference
You can listen for these events:
"documentNew", "documentOpened", "documentClosed", "mouseMove", "documentChanged",
"layerChanged", and "frameChanged".
for example:
fl.addEventListener("mouseMove",onMouseMove);
function onMouseMove(){
fl.trace("mouse moved");
}
You should also check out xJSFL. It's pretty handy when scripting Flash.