"In the short run, the market is a voting machine, but in the long run, the market is a weighing machine." -- Benjamin Graham

Reading Graham's quote today, it's apparent the populace is voting overwhelmingly in favor of a continued market rally, but the real question to ask is, "When will the market weighing machine vote 'no' and send shares tumbling?" To attempt an answer to this age-old dilemma, we'll look at margin account debt balances, which just recently hit a decade record high in April. In the following video, Fool analyst Blake Bos explains what margin debt is, whether these debt balances can predict a sell-off, and how he's placing his investment bets in today's heated market.

Blake Bos owns shares of Apple. The Motley Fool recommends and owns shares of Apple, Google, Netflix, and Walt Disney. Try any of our Foolish newsletter services free for 30 days. We Fools don't all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.