You struggle toward the end
Ideally, the final few reps should be a real effort. The goal should be to push yourself until you are about 2 reps short of failure, or a rate of perceived exertion of 8 out of 10.
You’re able to recover quickly
After a high-intensity set, your heart rate is up and you’re out of breath. Recovery should be fairly efficient, leaving you ready to continue your exercises. If you have trouble recovering, you’re working too hard.