You got the right point. a market manipulator will never admits about his/her cheating, but it's not rocket science to understand that market is looks manipulated or not, it's very simple calculation, in a project of course a significant amount of tokens holders from team or big investors, so they have power to manipulate in the market if they just go to sell their all tokens, it's called manipulation, it's absolutely possible, don't need to see other evidence.
It is highly possible for whales to manipulate market but not just any market but those altcoind market. That is why it is good to go for a better coin. Bitcoin is having a marketcap oof over a trillion dollar which can be more resistable to market manipulation. But many people will trade altcoind and still be complaining if market manipulation. Then why are they not treading with bitcoin instead of gambling with altcoins.