There is no direct way to find it but few points should come in handy.
One can keep an eye on open interest and if it is increasing with falling of prices then its a good sign to confirm build up of shorts.
Also any increase of activity of share pledging by promoters.
Any big business decision or lawsuit or corporate action is awaited along with increase in open interest and volatility is only a scenario to monitor carefully but not definite indication of build up of shorts.
To mention figuring out of building up of shorts is not a science and is more of an art.