it's partially feelings, sentiment on the forum, twitter, daytime, news ... not always you are really able to know, but time to time you just know.
And of course if you are aware and see your dumpfest about to begin, just jump in and ride
Bigger waves lasts more than 30min so at least one-two candles you can catch without much problems.

As I said, I don't care about catching very bottom or very top, but for monero these days it's around 10usd up/down on daily basis and this is enough to ride on.

If you miss it, no need to be bitter, It's part of the game. I see very very very often people doing a lot of lines in the charts predicting future ... for me it does not work nor make much sense as history proven that this market is very different from stocks.
90% of my investments are mid-long, short ones are basically just for fun, but when it's very obvious that something will happen I'm daytrading with some funds as well.