The counterpart of uninformed or retail flow, which is what market makers actively want.
Why internalising retail flow is valuable: it is uninformed, so the adverse selection cost is low and the spread can be captured more cleanly. That is the economic basis of payment for order flow.
Detecting it in real time is much of a market maker's technology: fill rates, the direction of subsequent price moves after your fills, and one-sided pressure. A rising loss per fill means the flow has turned informed and the quote must widen.