There isn’t really any way to be 100% sure that a rising price in an uptrend has finished rising or a falling price in a downtrend has finished falling. But the most likely next thing that happens in a trend is that the trend continues.
So if price goes through 70, 80, 90, 100, to 110, then falls back to 105, its probably in the very near future going to go back to 110 and then on to 120. So buying at 105 is a nice discount to the latest high price 110. In practice, you can reduce risk by not buying at 105 but by setting a buy order at say 106 or 107, so that you will only be long after price has already proven it is being driven higher again.
Almost all trends have corrections, they never move in straight lines upwards or downwards. The longer the time since the last correction, the more likely the next correction is to occur. Sometimes the time intervals are very regular. A correction becomes quickly more likely if price suddenly accelerates beyond a slow smooth trend.