Over the years I have noticed that most alt season started as follows:
1. $BTC pumps first.
2. $ETH $BET & $LTC coin pumps next, leading indicators of the Alt season.
3. Big marketcap tokens next. Usually top 20 on CMC.
4. Then full blown Alt season.
This procession is the way it happened around 2017. I remember around 2016, litecoin was very cheap to less than $5 but by the ending of 2017, it went around $60 and above. Likewise others, etheruem too came to ATH and many others. The movement always starts from bitcoin as it has happened. The altcoins have started too and market cap is rising as well.