Most whales like to brag about how many coins they have, so you may want to see what they say on Twitter.
Whales do this? I'm pretty sure most of them would rather blend in the background to avoid being a target. I'm not saying whales wouldn't brag, but most of the noisy ones are probably not real whales. Pay too much attention to them and you may be used for a pump and dump scheme or something similar.
There's really no definitive way to determine if a whale is going to be dumping is coins or investing.
This. Whales can make money by moving the market, and it would be against their best interests if the sheep knew when to HODL or dump. If they're making obvious moves, they're probably manipulating you. Roger Ver took advantage of this by moving his coins to an exchange when the original Segwit2x was cancelled, leading more people to sell amidst fears of a price slide.