Earnings increasing & multiples expanding (Re-rate commencing), page-7

With these stocks you can tell with relative accuracy between retail and institutional trading. For example repeated selling of $150K orders at a certain price point (3.3/3.4/3.5) in single orders is typical of an insto exit, as opposed to small $5-30K orders that is typical of retail activity.

There is no other way other than experience of watching a stock every minute of a trading day and every singe trade in course of sales to determine this. Often it could be a group of sellers or buyers, although personified as "the seller".