This is a huge, but probably unanswerable question.  But it is one that anyone smart is always hoping to find the answer to.

Markets are all about confidence and sentiment, so (sadly) there is no perfect answer, even though with hindsight patterns do repeat.  But patterns can always be seen in the past and look obvious - seeing them coming again is not so easy.  Like Elliot waves; it's always debatable 'which wave' we are in.

For long-term trend prediction (the big turning points), I think indicators of irrational bullishness /bearishness are as good an indicator of anything a crash, or rise is coming - but this is unmeasurable mathematically.

Short term - personally - I draw lines on charts endlessly, decide my favourite candle time scale and which MA to trade on and it usually 'helps' - but only enough to give me an edge.  And it's an edge that works only if you work hard and spend a LOT of time on it.

I did sell off 20% in Dec, which was as much as I dared to because it was enough to change my life significantly - and I didn't know what the tax situation in my country was likely to mean for what I took out (plus it was still going up!).   I felt strongly we would probably correct, and I remembered 2013, so I took 'some' out.  I advised members of my close family who had anything to do the same (they chose not to).

I will say this:

With trends (not short term trading), the only thing that has worked for me is two methodologies:

1.  Gut feeling  (along with mentally preparing to 'sell to too early, if it's enough money to make a difference')

2.  Listening to Tera.