The above chart comes from a the latest interesting piece by Doug Short on Advisor Perspectives. You can click the graph above to read the article in full which suggests that the build up in margin debt to extreme levels and the subsequent down turn in these debt levels can be a precursor to a market correction. Certainly the US market has had a great run and people have been starting to worry about valuations there with the CAPE (10 year) PE and other valuations measures getting towards the top of their ranges.
Talking of valuations and further to my post from the 1 April this year entitled Crazy Caped Crusader idea which suggested buying Russia as it was one of the cheapest markets on a CAPE (10 year) PE basis. I note from an article on Bloomberg recently that Russia has been the best performing market for the last couple of months and the ETF I suggested is up by 12% since then so perhaps it wasn't such a crazy idea after all, although I guess you'd have to be a determined contrarian to switch US equities into Russian equities?