I agree with Jordan. Since 2008 - and with all the regulations since then - trading on margin is a more tightly regulated business.
That being said, I have no doubt that people - mostly market-movers with a ton of dough - will ALWAYS find a way to game the system. BUT, I highly doubt that much margin is being used to fuel the market rise.
As a former FINRA-license adviser, I can tell you of the $$$billions that are still sitting "on the sidelines" since the market crashed in '08/'09. If you had just a single-digit percentage gain of investments from savings, CDs, bonds back into the stock market, you'd see this sort of increase.
Now...the bigger bubble I'd watch out for is the bond market. If stocks maintain this higher level through a few quarters, and people get dividends that pay better than 1.5%, then you may see a flight back into the market...