As Pete alludes to in his post above you have a statistically better chance making the working assumption that it's a pullback than a reversal,
providing price is indeed in a trend.
What you would be advised to do first is identify what you mean by a trend and how many t/f's it exists on...for eg I tend to use opa (overall price action analysis - peak/valleys) to identify a 'classic' trend. and if this trend exists on t/f's say up to 4hr then there is potentially a strong trend.
When pricce starts to move counter to that trend it will be either be;
a. a pullback
b. an extended pullback
c. a reversal in trend.
Dealinng with each in turn;
a. A Pullback
This will typically be shallow and if you are trying to time the best entrry back into the trend after a pullback then you have to determine which trend - ie using the example laid out above of a claccis opa trend to 4hr, will this be the 5min trend, the 30min, the 1hr trend or the 4hr trend ? For eg price may be trending on t/f's up to the 4hr but the trend starts on the 5min, so I would be looking for the first potential sbr/rbs area on 5min and seeing if any 1min PA etc supports a re-entry to the trend there on any pullback to it, if not then we have an extended pullback;
b. Extended pullback
This will typically be lengthier than a pullback and carrying on with the example above if the 1st obvious potential sbr/rbs area on the 5min failed to hold the pullback and it is extending deeper..so I look for the potential sbr/rbs areas on the 30m/1hr t/f at which I could time an entry back into that part of the trend using 5/15min PA etc... if this fails to hold then I'd be looking to identify potential sbr/rbs on the 4hr and looking to identify 1hr PA etc there to get back with this part of the trend.
Good example on cable recently..price pulls back from last week's/friday's hi in an uptrend that extends to the 4hr..price pulls back and the 1st 5min then 1st 30min/1hr obvious potential rbs fails to halt the pullback but at the previous 4hr swing hi (confluence with 50% of the a-b move shown) we get some 1hr PA that suggests that this may be a good place to get back with the 4hr uptrend, a good 1hr classic hidden divergence set-up there too as well as other suppoirting set-ups by which to time an entry on t/f's below it.
c. A Reversal.
Each potential sbr/rbs on successively higher t/f's that form part of the trend that are not respected by price as price continues to pull back means that there is a greater chance of an opposing trend forming on those lower t/f's and if the last potential sbr/rbs fails on the highest t/f that forms part of the trend then this probably means that the trend you are trying to enter may be over in favour of opposing trend s on the longest t/f it existsed on and/or below - particularly if opa as described above supports this -or- price may just start to consolidate/range.
As to whether there will be a pullback or extended pullback or more well things to look for here include
a. If there is any opposing regular divergence at the highest point of the trend and how many t/f's this extends on
b. the potential strength of the potential supp/res that has caused the pulback or more
c. Price's relationship to it's atr's.
There are no absolutes, just set-ups, analysis of opa conditions and working assumptions. You stand a better chance of guauging wether it is apullback or reversal though if you apply some logical methodology to your analysis.
G/L