Basically you can't tell unless it is an illiquid stock where you have been to the register and copied down the number of shares held by each shareholder. When that number comes up for sale you know who it is. On the other hand if I am moving shares in an illiquid stock I will sell in parcel size that reflects someone else s holding. The only way you can usefully know takes considerable effort. You have to inspect the Register regularly once per month. It will not take into account the short sellers who " borrow " stock which is readily done. For your education go to the DOM announcement 10.12.10 by Credit Swisse 84 pages long. If you look you can see they were trading the stock from Aug to Oct [ many + and then - sales ] because it had plenty of liquidity. When the KCN merger was announced they switched to being an arbitrager -- there is about a 3 % disparity in the KCN v's DOM price and they are narrowing the difference.It's a risk free strategy as they ultimately fold the DOM shares into KCN at a known rate. Go to the last 10 pages of the announcement which basically says -- we borrow the money -- we borrow the shares so we can short sell if we want to -- then we rely on our trading ability to make a dollar.
