There are no hard-and-fast rules for determining that. Just as you said, I usually look at T&S. Sometimes, those large block traders will do head fakes but posting huge blocks at the bid as well as the ask prices so as to create confusion. Sometimes, they don't of course. And if they do and when they are accumulating, they usually set ridiculously high ask prices so that nobody will buy them. They just want to create panic retail selling so that they can buy the shares at cheaper prices. On the other hand, if they are distributing, they will make huge size bids at ridiculously low prices so that nobody would be willing to sell to them. They want to create a panic buy so that they can unload the shares. But, I still find reading the charts to be more reliable, because at the end of the day, if there is sufficient accumulation or distribution, it will show no matter how much they head fake in the course of the day.