Can someone suggest me where to find an indicator to see which are the most and less volatile pairs in forex market on different periods and market conditions?
I'd use average true range or standard deviation. Average true range is pretty good and standard deviation by definition is volatility.
Average true range and standard deviation will pretty much give the same picture.
Keep in mind that they're lagging indicators in that they are calculated on a past data. The best course of action is to look at the cyclicality of the indicator (what times of day it peaks) and then understand that right before the peak begins to form is when trading volatility is best.
Most platforms have 1 of these 2 indicators. I'd start with a 15 - 30 minute chart to see the times that volatility peaks.