There is an old indicator that I have been paying attention to for choosing altcoins for several years - Bull Market Percentage (https://www.turtlebc.com/tools/bull_percentage/btc?from_date=2014-02-11&to_date=2019-08-16&exchange=all). The more altos from the entire market are at the pump stage, the stronger the percentage indicator grows. And vice versa, the indicator falls along with the fall of the bulk of the coins. Counting is done in #BTC, not in dollars, this is important. Today, the indicator fell to an absolute minimum - 1%. Statistics has been going on since 2014, and there were only three such lows in history, all of them happened in the same 2014.
I'll say from personal experience that after such low marks, a trend reversal occurs literally in a few months. This does not mean that altcoins have reached an absolute bottom. But we can say for sure that buying them now is much more rational than during the growth of the indicator to the level of 50-70%.