AFTER EFFECTS // text layer display name of current visible layer

http://www.motionscript.com/design-guide/source-name.html

txt = "";
for (i = 1; i <= thisComp.numLayers; i++){
  if (i == index) continue;
  L = thisComp.layer(i);
  if (! (L.hasVideo && L.active)) continue;
  try{
    txt = L.source.name;
  }catch(err){
    txt = L.name
  }
  break;
}