Here, I have analysed the chart of Sensex, where we see a few things :
Firstly, a double top is forming very soon ( the white line )
Secondly, the yellow trendline has been very well respected till now.
I have drawn vertical lines to illustrate that.
You can very well see that price has reacted very well and except for few false piercings, it immediately took support on it. But in Jan 2016, it went below the line strongly and prices fell for a few weeks and later recovered. Again in Sep 2016 when it tried to cross the line, prices fell and continued downwards for a few weeks. And then we recovered.
So now, we are very close to meeting that line again and possibly halting the bull run for a while.
While I won't advise to build short positions now itself, do keep this chart these facts in mind to manage your positions well :)