U.S. stocks are trading near record highs heading into the thick of the second-quarter earnings season, but with markets historically expensive and reliant on the performance of tech companies, some investors are starting to embrace a more cautious outlook.

With Wall Street's surge to record highs, the U.S. stock market looks nearly as expensive as ever, and investors are debating whether the lofty valuations are a bearish signal or justified by the technology-heavy market's profit outlook.