just look at coinmarketcap, if the majority of coins decline by more than a few percent, that means we are entering a bearish trend. or more difficult, you can look at the ma and see what the pattern is, or use other trading tools that you trust, you can definitely see a bearish trend in the market.
but it is quite difficult to identify whether the market is truly bearish, or whether it is just a momentary market response, because in some cases the market drops a few percent and people have concluded that the market has entered a bearish trend, but the next day the market rises significantly .
that's why traders need to analyze the market in depth and learn from their experiences, because it will really help them to be able to identify whether the market is bearish or bullish.
That is just the start of it, the idea to learn "before" it happens is the main question I believe. I do not think that it is possible and I think it is not going to happen, nobody could learn a way to make sure that they know when the bear will start, there isn't some indicator that will tell you before it starts.
However, if that is not the question then yeah you are right, if it starts to drop, it is better to be careful. I also use plenty of methods to avoid, like stop loss for example, and trailing stop as well, so there are many other ways as well, it should be fine. I get that it is not going to be easy to not lose money ever, so just realize that you are going to end up losing time to time and you will be fine in the end.