In the allegory of the cave, Plato views the sunlight as the truth, and the shadows in the cave as being false, and his contribution to the question "how can we tell what is true, and what is false" is that we have no way of knowing what is true, and what is false, until we have experienced them both, and can compare the two. I think that Plato is trying to say that society is ignorant of the truth, and prefers to look at the shadows on the wall, and we must try to break the bonds of those around us in order to see the truth of sunlight and the real world.
I think that Hobbes believes that all life, even spiritual, is made up of matter. He believes that material is real, and everything else is false. For example, he believes that spiritual substance is a derivative of material substance, which makes spiritual substance real.
Descartes contributes his thoughts such that, in the waking world we perceive things around us that are real, and although our senses mislead us once in a while, that doesn't change the fact that the things around us are real. He compares this to our dreams, and how we can be mislead into thinking that we are someplace that we are not. Descartes notes that both things we perceive in the waking world, and things we perceive in our dreams are formed in our minds, so then how are we sure that what we see whial awake is real?