Well Mr MN, I find you quite patronising and lacking imagination. Just because accidents and violence which result in heavy blood loss are not common events, that doesn't mean that I, or anybody else reading this, hasn't seen it happen, or been involved. Certainly blood has a way of seeming like more than it actually is and, if someone turns up when it is already on the ground (in the clothes, covering the hands, spattering faces, on the foliage, etc), it may indeed be difficult to tell 3L from 1L, however if even the dopiest person to ever wield an AEG came across somebody with bright red blood spurting out of a wound around the fingers of the desperate casualty, they would very quickly be able to tell that if blood is not prevented from spurting out quite soon, the casualty will die.
It doesn't matter how much is already out, it's the rate at which it keeps coming out and the state of the casualty. If s/he has blue lips and grey skin, is out cold, and the blood is losing power in how far it sprays, it is clearly because no matter how much has come out, it is too much. At which point the urgency of the situation has gone from "Shit, I need to do whatever I fully understand to try to stabilise him/her until paramedics arrive," to "Shit, unless someone can tell me for sure that paramedics will be here in 3mins or less, we need to do whatever we can, because otherwise it won't matter."
Of course it helps if you have picked up tips from healthcare professionals, like if you pinch the skin immediately above the biceps hard and there is no change in pallor nor slight reddening around the imprint when you let go, the casualty has dangerously low blood pressure (and that works on people with any skin colour), or if someone is unconscious you can find out if that is because they have passed out due to fear (erroneously referred to as 'passing out from shock'), or whether it has resulted from damage caused by the injury, by rubbing your knuckles hard up and down the breastbone - it hurts enough to bring you round if you can come round. If you can't get at the breastbone, you can stick your fingers into the armpit and squeeze the pectoral muscle with your thumb to get the same wakeup pain effect. Or my personal favourite for when performing CPR, "If you don't break at least one rib, you're not doing it properly!"