Based on my experience most of the time they will go to loe volume coins then suddenly it will go up. Of course, these sre pump groups as well that has a large amount of capital than manipulate its market over time. Well at some point you just need to ride however theres a chance of loss.
Is there any warning? Which indicators help to judge in advance
Is there a reason of why you want to know if a whale is interested in an altcoin? Do you want to take advantage of the moment when the coin pumps or something similar?
And to answer your question whales are smart, they will try to get positions in a coin without leaving traces or evidence they are interested in an altcoin, if they left any traces then people will know of their involvement and they will not get their projected profits, so unless you are part of the circle of whales I doubt you will know beforehand in which altcoins they decided to invest.