"Bullish sentiment is definitely high," he added, citing a TD Ameritrade index on sentiment of both retail and professional investors.
"A lot of our traders are selling into the rally and buying on dips," he said. "The long-term investors on our platform recognize that if you were out of stocks and went into cash in 2009, you missed out on one of the greatest rallies in the history of financial services."
One traditional measure of investor appetite and sentiment has been margin accounts, which allow investors to leverage their bets by borrowing money to purchase stocks.
"Margin balances are relatively flat," Bradley said, estimating that out of $500 billion in assets, investors on his platform own about $8 billion to $9 billion on margin.
(Read More: What the Pros Are Saying About the Market's Rally)
However, he noted, "cash levels are at highs, in excess of 20 percent, on average, on the retail accounts. Professionals have much lower cash levels." Bradley added that "one of the worst things you can do is sit on the sidelines and not participate at all."