How do they know if you need transfusion?

If you lost blood. Transfusion is reserved for cases where blood is lost and needs to be replaced. Patients can do this before surgery by autologous transfusion, where blood donated from a patient is returned during surgery, or in anemic or emergent cases, like trauma victims. These patients have lost blood and need blood fast, and are often in shock, so the best way is to give them type matched blood.