It's homework you have to do for every pair you want to trade but you can use these zones for a very long time when market moves sideway. Low prices attracts buyers and high prices sellers. Since humans like numbers chart key points are often searched in history and this forms areas.
When the market enters these areas and bounces off immediately producing long wicks it means an aggressive trader from another timeframe entered the market. It's a good hint something has changed and that you should reposition yourself unless you want to be a straggler. The more long wicks in a row the (exponentially) stronger the sign of a change - sometimes it looks like a beard. If you need an indicator use a momentum indicator or Fibo levels.
