Which of these methods would be most effective in allowing someone to precisely gauge the intensity level of his exercise?

You didn't really provide us with any possible list of methods. But to know how intense your exercise is, you would usually use the maximum amount of your heartbeats in a certain period to see how taxed your cardiovascular system is. This is also a good ballpark that tells you how intense your exercise is. 

Stimulants speed up the central nervous system.