At the end of each training session, there is a gauge to measure the intensity of the workout, from a scale of effortless to beyond hard. While it might seem obvious on how to judge this, I want to focus on the three specific factors to examine when doing so: effort/burn/pump.
EFFORT: When training, there are demands put on the body. This includes the CNS (Central Nervous System), along with the cardiovascular and of course muscular system primarily. Three sets of leg extensions is a much less effort than the same level of squats on the body on these systems. By ranking on a scale of 0 to 3 (3 being the highest effort level), you can calculate the level each training took.
BURN: While lifting, waste products accumulate in the muscle (also called metabolites). These are byproducts from muscle contraction and are part of the fatigue process. It often is described as the "burn" you feel in the muscle and can be a trigger of muscle stimulation/growth. Again, measure this on a scale of 0-3.
PUMP: This is likely everyone's favorite, the level of blood engorgement in the muscle. Full muscle bellies, veins popping, a good pump is a sign of an effective training session. This can also be a precursor to growth/hypertrophy. While it dissipates over time, those moments following a set usually are the highest. Rank the pump throughout the training session on the same scale.
Utilizing these three factors, hopefully you can better accurately judge and measure the level each training session is on. The body reacts differently to each movement and paying attention to these will help you get the most out of each training session.