Asset bubbles are notoriously difficult to identify as they are happening. Often times, they only become clear in hindsight.
Having said that, Goldman Sachs' David Kostin offers this interesting stock market chart in a new note to clients. It's a favorite of his.
It shows the sector composition of the S&P 500 by market cap since 1975. As you can see, sector bubbles manifest when they suddenly explode as a percentage of the S&P 500.
The dotcom bubble is very prominent, represented by the ballooning info tech sector stocks. The credit bubble appeared much more gradually as seen in the rise of financial sector stocks.
Does any particular sector stick out today? Maybe we'll see in a few years from now after it collapses.
Goldman Sachs