The Bitcoin Dominance Chart (BTC.D) allows you to understand how much of the entire crypto market is made out of Bitcoin's Market Cap (Value).
The start of a Bull Run will be shown with the chart increasing which is showing Bitcoin is gaining more value than Altcoins in the market.
Using the understanding of how money flows in cryptocurrency, the flow begins with Bitcoin increasing, followed by profits from Bitcoin flowing into Ethereum and finally flowing into other Altcoins.
As soon as we reach the middle of the Bull Run, we can use technical analysis to identify when the charts will reverse and Altcoin season will start. At that point, moving your investments from Bitcoin to Altcoins would be advised.