This python script can be used in QGIS actions. It allows to run a specific code on the very las feature when clicking on a stack of features.

QgsDetectLastFeature.py

from PyQt4.QtCore import QSettings

class Selection(QgsMapTool):
	def __init__(self, canvas, layer):
		QgsMapTool.__init__(self, canvas)
		self.canvas = canvas
		self.layer = layer

	def getFeatureRequest(self):
		sr = self.searchRadiusMU(self.canvas)
		r = QgsRectangle()
		r.setXMinimum([%$clickx%] - sr)
		r.setXMaximum([%$clickx%] + sr)
		r.setYMinimum([%$clicky%] - sr)
		r.setYMaximum([%$clicky%] + sr)
		r = self.toLayerCoordinates(self.layer, r)
		return QgsFeatureRequest().setFilterRect(r).setFlags(QgsFeatureRequest.ExactIntersect)

	def getFeaures(self):
		return self.layer.getFeatures(self.getFeatureRequest())

	def getLastFeature(self):
		feature = QgsFeature()
		features = self.layer.getFeatures(self.getFeatureRequest())
		for feature in features:
			pass
		return feature

	def getLastFeatureId(self):
		return self.getLastFeature().id()

def qPrint(message):
	txt = unicode(message)
	if txt:
		QgsMessageLog.logMessage(txt, tag="pics", level=QgsMessageLog.INFO)

def getLayer():
	return qgis.utils.iface.activeLayer()

def getCanvas():
	return qgis.utils.iface.mapCanvas()

def getFeature():
	feature = QgsFeature()
	features = getLayer().getFeatures(QgsFeatureRequest([% $id %]))
	features.nextFeature(feature)
	return feature

if __name__=='__main__':
	selection = Selection(getCanvas(), getLayer())
	currentId = getFeature().id()
	if selection.getLastFeatureId() == currentId:
		qPrint('This is the one at the top: ' + str(currentId))
	else:
		qPrint('This is NOT the one at the top: ' + str(currentId))