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.