Ok, I'm not saying anyone (especially if they don't have proper knowledge) should diagnose and treat their dog. But its always good to know what your dog
might have, especially if you are thinking if could be a number of things. The following link is handy in that you can click on what kind of symptoms your dog currently has and then get results of diseases that have similar symtoms. By clicking on the disease you will find an article full of information on that illness and help educate yourself.
Of course with serious illnesses you will need a vets help so don't ever forego that. And if the vet's diagnosis is different then what you got, don't argue and say that a site told you something different. Although you can alway inquire if there is a possibility that it might be a disease you researched, its a chance to learn even more.
Anyway, here's the link:
http://www.doggedhealth.com/symptom-selector.html