The quest to find whether cryptocurrency bitcoin is in a bubble could be found on search engines, where one bubbly sounding term is on the rise.
Google Trends says the search term "buy bitcoin with credit card" is around its historic peak, notes Nick Colas, co-founder of Data Trek Research and the first Wall Street analyst to take the digital currency seriously.
While the phrase is only about 3 percent of all bitcoin searches, it is on the rise.
The popularity and curiosity of using leverage to get in on the action come as bitcoin's price continues to scale new heights. In another surge to the upside Monday, bitcoin rose past USD 9,600 in morning trade, a rise of nearly 7 percent in a 24-hour period and 17.5 percent over the past week alone. It later was trading around USD 9,500.
"These bubbles tend to end in tears," Citadel hedge manager Ken Griffin told CNBC in an interview Monday.
Colas sees the inclination to use credit to get in on the moves of a volatile asset as "a cautious note" about where sentiment is running.
"For those readers who think about transmission mechanisms between financial assets and the real economy, look no further. This is it," he said in his daily note for clients.
Other search trends show that bitcoin holds a solid lead in Google quests for digital currencies, even though it has a growing list of competitors. In all, 15 cryptocurrencies now have market valuations above USD 1 billion, according to CoinMarketCap. At USD 162.4 billion, bitcoin is more valuable than the next 70 or so competitors combined.
Colas said bitcoin "still holds a strong lead in terms of global attention in crypto-land."
Bitcoin also has come in from the shadows in terms of the attention it's getting on Wall Street. Investing heavyweight Michael Novogratz, head of Galaxy Investment Partners, thinks bitcoin will hit USD 10,000 before the end of the year, and Tom Lee at Fundstrat has a mid-2018 price target of USD 11,500, nearly double what it was in mid-August.
But Mitchell Goldberg, head of ClientFirst Strategy, told clients he's sceptical and sees the bitcoin craze as analogous to the dot-com bubble and a sign that "we've entered the final and stupidest part of the bull market."