In practice
Markets look ahead. When traders expect a rate cut or a strong result, they buy in advance, so the price already contains that expectation by the time the event arrives.
The useful question before any event is what the market already assumes. The opportunity lies in the gap between that assumption and what actually happens.
What to watch for
This is why good news can be followed by a fall: the buyers were already in, and the event gives them a reason to take profit. Nobody can measure exactly how much is priced in, so it remains a judgement.