Thoughts on judgment

When we make a judgment on anything, ie, the legitimacy of an argument, the value of someone's word, an explanation we hear, an interpretation of an historical event or just about anything else, what criteria are we using? It seems to me that most people would appeal to "common sense", "public opinion", or "that's how we have always done it". 
But if we give this much thought, we can see just how shaky this ground is.  Do any of us, for example, have enough understanding of economic theory to validate or critique the position promoted by any one particular presidential candidate?  Do any of us have clear understanding in the field of medical ethics or even medicine in general to argue questions about euthanasia, abortion or GMO's?
In addition to these, how many of us have a good understanding on history and the mistakes that have been made in the past?  Enough to see those errors being made again, with just slightly different circumstances or details?
I am not going to offer an opinion or answer to this.  I merely wanted to bring up the thought and ask, ultimately, to whom do we look for answers and guidance, and why.  I am afraid that ultimately, the final authority we choose is ourselves.  When it comes down to it, we choose the answer that seems best to us, when honestly, we have the smallest and least informed grasp on topic x, y or z.  Who is our final authority?