In my opinion, nothing is "reliable" in forex. However, more to the point, I don't think there is any way of identifying the exact point when a trend has started. You can only identify the trend after it has already started. As some people have mentioned already, there are multiple indicators that can identify this and you can also see it with your naked eyes by just seeing whether the price is forming higher highs and higher lows. Of course the concept of a trend itself is also subject to what time frame you are referring to. Every trading range is actually an up and then down trend on a lower time frame over and over again. To a day trader the price may be trending, but to a swing trader the price may be caught in range. It all depends on your point of view. So back to your question, typically the fastest way of identifying the start of a trend is a breakout from a trading range. Obviously this approach is not "reliable" due to false signals, but it is one of the earliest ways of jumping into a trend if one is going to start at all.