For Rana temporaria newly hatched tadpoles, this metod worked: if you shine a light on the tadpoles, dead and alive would seem to have different colors, alive would be dark and there would be no real difference in their color when shined and not shined, dead would turn into "pale" under lightsourse, they would really look like "dead". The difference is significant and you for sure will notice it. In complicated cases I used long and slim wooden bar (any slim ellongated thing works) and carefully touched the tadpole. Alive would reveal him by moving. Sometimes he moves only after second or third touch. Obviously, dead won't move even after tenth touch. This touching also showed me that "shining" method is extremely reliable, but I'm not sure if it works fine for species other then rana temporaria.