Blood loss during major surgery is best estimated by:
Measurement of blood loss during surgery (Gravimetric Method)

Blood loss during operation is measured by: Weighing the swabs after use and subtracting the dry weight and fluid used + volume of blood collected in suction bottles (after subtracting irrigating fluid)
Visual assessment
Suction bottles
Transesophageal USG Doppler
Cardiac output by thermodilution