Dow theory is to identify the trend using minimum of 2 years chart , it is used to find the demand and supply
Demand > supply then it is the bullish trend
Supply > Demand then it is the bearish trend.
i.e In Bullish trend we will get higher tops and higher bottom (current bottom is higher than the previous bottom). when higher top is breaking with volume then it is actual resistance got broken. Trend reversal for bullish is when lower bottom formed.