That's the point of an article by Jordan Roy-Byrne, who says investment professionals are leery of recommending gold for three reasons. First of all, it's harder for an investment pro to make money by recommending gold. Secondly, most investment advisors see gold's rise as abberational; the '80s and '90s markets are seen as the norm. Thirdly: with two bubbles popped this last decade, the rising gold price has made them skittish; consequently, they're prone to see bubbles in any high-flying investment or asset.
The rest of his piece discuss four charts suggesting gold is not in a bubble, gold is underowned if anything, and gold stocks are on the cusp of a further rise. Two of his charts are ratio charts: the first measures gold in terms of the S&P; the second, Barron's Gold Mining Index over the 500. Both are closer to historical lows than historical highs.
Of course, there's no guarantee that a ratio chart is stable enough to provide insight into the future. The S&P 500 has gone up more than ten times in the last thirty years because of overall U.S. economic growth. The denominator expanding over time due to long-term growth does introduce a distortion.