There isn't a way to know for sure when an activist investor is selling a stock. However, since activist investors own a large portion of the shares, selling them all at once would put significant downward pressure on the stock price. If someone notices the stock price suddenly drop with massive share volume, that could be an indication that a major shareholder is selling off their stake in the company. Even if the selling took place over several days, a major shareholder like an activist investor could still drag the stock price down.