How do you spot it? One way is a swing in the opposite direction and a test preferable touching the previous trendline. What are other known ways?
your question is rather interesting for what it does not ask :because though there are a few trend reversal signals, what is much more important is
till where the reversal's ultimate destination is or will be.
without this information it is like getting on a bus or train without knowing it's ultimate destination.
A better question
will be if a reversal signal occurs what is it exactly reversing,because that will be the target:if the reversal signal is reversing a one bar spike then the market will only go as far as the bottom of the spike.
In a strong trend you will see many reversal signals that do not go beyond the extreme of the bar and
"fail";in reality they do not fail because
that was the target for the reversal signal
Equally pertinent is where the signal takes place: if it occurs in channel, then the destination will be the extreme of the channel.
Since the market is always in a channel,sometimes the channel is so steep, it is called a spike or trend,
most reversal signals result in what most traders would call insipid moves, and this has led some 'gurus' like Al Brooks,to say that most reversal signals fail.
This is totally wrong because the target was achieved and is always achieved.
A reversal signal does
not usually result in
big moves and the preoccupation of traders in identifying reversal signals suggests that many or most traders believe this is so.