Rates of indebtedness, quality of debt(AAA, BBB ratings etc), stock market overvaluation, real estate overvaluation. Take your pick, conditions are very similar to 2007. Whether anything actually happens is the trillion dollar question.
This was recently posted on HN and gives a good overview of indicators to look at though it's pretty blatantly alarmist.
Makes me definitely consider putting this housing search on the back burner. In my area we've seen massive price increases in just the last 3 years. I was on a tour of a home yesterday and the agent told me that the previous home tour was with a bay area investor who had cash on hand for the home.
Hard to compete with that. And maybe it's just the wrong time to be competing at all.
Buy gold?