This is easy to identify in some cases and very hard in other. That dump on January 23 was a whale move because how would you explain it otherwise? The price went down by $300 in just 30minutes  and somebody dumped 400BTC on Bitstamp in 3 minutes instantly taking the price down by $120.

It gets harder when that whale buys or sells many times a day putting pressure on the market but not dumping at all. You will never be sure who is doing what because exchanges will not disclose their users.