If nothing else, one can examine the current expansion's duration against the history of other expansions.

The longest business cycle expansion in U.S. history lasted 119 months. The current one is going on 97 months.

Even in the absence of "signs", one could make a rational bet that we are much nearer the end of the current run than the beginning, particularly with the headwinds of rate hike expectations.

Unless, of course, it really is different, this time.

The article cites prices drifting ever higher with low volatility. Even as a card-carrying market prediction skeptic, I think this has to be seen as worrying because it implies the market is detached from the usual random news events that should each cause price movements. Certainly when there are no corresponding "you've never had it so good" economic conditions.