There is a saying from Sir John Templeton that "Bull markets are born on pessimism, grow on skepticism, mature on optimism and die on euphoria."
I think you can use Fund flows as a guide to that sentiment. The chart below shows how much money has gone into bond funds versus equity funds since 1984. What's striking is how much more has gone towards bond funds since 2009.
Looking at this it's hard to believe we are close to euphoria. We may have pullbacks and corrections but if sentiment is a guide this market has room to run.