Most whales like to brag about how many coins they have, so you may want to see what they say on Twitter.
This would also bring up a lot of false positives just because a lot of people aren't 100% honest when it comes to their finances and if they do have a lot of coins, then they could be trying to influence the market themselves through their followers causing panic selling/buying.
There's really no definitive way to determine if a whale is going to be dumping is coins or investing.
Too risky to depend on any whales for your decision making cause we cannot say that they are telling the truth or they just want people to panic. For me better to make your own study, invest on great coins and whales will come for sure cause its a great coin and worth investing for. Technical analysis can help you decide when to buy and sell, just keep on learning.