What is the difference between confidence and overconfidence? People say there is a very small difference between confidence and overconfidence. 


How can knowledge of psychology help someone know whether they are are confident or overconfident? 
What is the state of mind when someone is overconfident?
Usually, confidence is based on experience; proven results with regards to a task or ability increase one's belief that the task can be completed a second time.  Notice that this has nothing to do with what others believe you can do - it's about what you believe you can do.

Overconfidence is bravado - false posturing to fool or mislead others; most often used to hide an actual lack of confidence.  This is all about getting others to believe that one is capable of a task.

There are a lot of non-scientific articles related to the difference between the two concepts:
Here is a "10 characteristics of" page from Hindustan Times
while here is an article from National Geographic talking about narcissism and overconfidence.  

These were found with a Google Search for 'overconfidence vs confidence'.  I'm sure you would get more scientific results if you dig some digging, or tried Google Scholar.