Answer:
When we exercise the blood capillaries dilate because of the removal of waste products like carbon dioxide from muscles during exercise.
The dilated blood vessels results in the increased blood flow and increased blood flow leads to the delivering of more amount of oxygen to the body muscles and cells during the exercise.
So increased blood flow is necessary for the removal of waste products like carbon dioxide and lactic acid generated during exercise and to deliver the appropriate amount of oxygen to muscles to function properly.
Not enough detail.
One way is the verbal Rating of perceived exertion scale 6-20
Another is heart rate range compared to their Max heart rate 220-age