How do you tell if dog is pale skin color from blood loss?

Lift the flap of skin at the mouth - and press gently (but firmly) with your finger, on the inside (pink) flesh. The patch of skin where you pressed should turn paler than the surrounding tissue. When you release your finger - if the flesh stays pale (instead of returning to its natural pink) - the dog probably has low blood pressure (a symptom of blood-loss).