good question..i think 50 million traders would like to know the same answer. no one knows for sure. watch support and resistance breaks or breaks of previous day high and lows and make your best guess where the market might go next. if the market breaks one of these barriers look for the next area of support or resistance for a possible range. other than that i'm not sure.
You ask the wrong questions. How do you spot the momentum to identify the exit and entry points into the trend should be your question.
Say the 'beginning' equals some random time such as 9:30am... then I guess the trend could possibly, maybe perhaps, there is a change that by 11:30 you can determine a trend.... well, right before the lunch lull, then the close rally/sell off thus breaking what we might of determined the trend was is now something new a different... A mantra of mine is "don't predict pivots, you don't know where the price action is going!" There is a 'but'... some technical analysis people could extrapolate, based on patterns such rounded bottoms and historical data, that the odds of the current price action represents a trend reversal thus a beginning of a new trend is greater than 50/50. Some traders spot the beginning of a trend with other "confirmation" techniques, such as, well, what ever they use.... but the good traders know the odds of their 'prediction' being completely wrong...... Others base it on fundamental data... interests rates, employment data, earnings, consumer sentiment and other factors..... Then lets throw a 3rd dimension on a 2D world.... time frames... ok, let's skip that.... So what does that leave us?