There is no definitive way to 'know' when a bottom is in place. That is why a good many traders will sell it all the way to new highs and beyond.....'it can't go much higher than this' type of mentality.
Take a look at my trend trading charts for a visual on buying the breakout of bull flags.
There are many other ways including breaking through....
*The high *The previous swing high *A key resistance price level *A moving average that has been resistance
I think it is better to simply buy the dip at a key a support level which is within my money management strategy.
...the list is endless really.
The important thing to remember is that nothing ever works 100% of the time.