Better players are more attuned to small mishits.
Missing the actual sweet spot can still feel good and work out. Some GI irons mask the slight mishits, and higher handicappers are less attuned. Double difficulty to detect.
I'll specify that, like @Pretzel, I want to practice like I play, but if I'm warming up I'll typically make overly fast swings without hitting a ball just to loosen up more. But when I hit a ball, it's the same shots I'd be doing on the course.
Question: How would I know if I have hit the sweet spot?
I'm thinking (guessing) I hit it one out of ten times, but when people who are much better than I am say they hit it less than that, I'm thinking maybe I'm mistaken, and it leaves me to wonder, how does one know that they have hit the sweet spot?
100%, but I'd specify that it's because I try to practice how I play. I don't try to swing harder or softer on the range or course for standard shots. For in between yardages on the course I might swing harder or softer with a club than on the range, or a bit harder on a drive if it needs to carry, but otherwise I'd say I swing at similar efforts.