The first sign of a crash. The Hindenburgh omen.

As reported in Zero Hedge

Despite the mainstream media’s desperate need to play down any and every potential indication that all is not well, the Hindenburg Omen signals are starting to cluster (in a confirming manner)

Via Wikipedia:

From historical data, the probability of a move greater than 5% to the downside after a confirmed Hindenburg Omen was 77% [The Wall Street Journal 8/23/2010 article states that accuracy is 25%, looking at period from 1985], and usually takes place within the next forty days. The probability of a panic sell-out was 41% and the probability of a major stock market crash was 24%. Though the Omen does not have a 100% success rate, every NYSE crash since 1985 has been preceded by a Hindenburg Omen. Of the previous 25 confirmed signals only two (8%) have failed to predict at least mild (2.0% to 4.9%) declines.