Corporate debt in the U.S. is now higher than it's ever been.

This is typically manageable if companies have a lot of cash to service that debt. But a looming problem, many economists are warning, is that — excluding the country's biggest companies — debt-to-cash ratio is now higher than it was in 2008 during the financial crisis.

Ten years on from the crash of Lehman Brothers that heralded the Great Recession, market watchers are looking for clues as to where the root of the next crisis might lie. Steve Blitz, chief U.S. economist at TS Lombard, sees a giant red flag in corporations' debt versus their means to pay that debt off.

"The real biggest problem lies, if I'm looking at the U.S., I look at debt-to-cash ratios, and I take out the top ten companies," Blitz told CNBC's Squawk Box Europe on Wednesday, noting his exclusion of highly capitalized companies like major tech and pharmaceutical firms who are well-stocked to service their debts.

"Debt to cash is very very high, but debt equity is very very low. What that tells me is corporations are borrowing against their net worth, as opposed to borrowing against cash flow and income, which in effect is the same thing households were doing in 2004, 2005 and 2006."

S&P Global reported U.S. corporate debt at a massive $6.3 trillion in June, with companies also holding a record $2.1 trillion in cash to service that debt. But most of that cash is held by a handful of giants at the top.

Meanwhile, riskier borrowers are more leveraged than ever before.

The cash-to-debt ratio of speculative-grade borrowers reached a record low of 12 percent in 2017, below the 14 percent level in 2008 — meaning that for every dollar they have in cash, they have $8 of debt.