My choice is the MZM (Money Zero Maturity) aggregate since it is an up-to-date ready-made measure of the volume of liquid money in the economy. Other measures, such as the popular M2, do not work this way.
In spite of popular theories of the American stock market being in the bubble stage, it does not look like that. The inflation-adjusted Dow is at the same level as it was in the autumn of 1994, two years before the famous Greenspan's quote about "irrational exuberance."
However, stocks are as expensive as they were in 1987 just before the painful crash in October. Thus, even though the market is not in the bubble phase yet, anything can happen.
Disclaimer: As you probably know, this is no offer or solicitation to buy or sell any securities, and everything you do with your money is at your own risk. Honestly, you would be crazy if you acted upon a piece of information you happen to find on the Internet.