How do you know your current strain during a strength training session.

The main reason I use whoop while working out is to avoid overstraining. Days when I am in the green I would want to go hard but days when I am in yellow I would want to stay below my recommended strain number. The issue is that while I am in strength training I have no way of finding out what my current strain is at.. leaving me me guessing if I should go harder or not. Is there anything I am missing? Is there a way I can actually tell where my strain is at during a stench training session?