Unless they watched you go from chair to pool there is no way to know. Thus another reason to not assume someone is a chair hog and move their stuff. I often will go to the pool alone and have never had a problem with people moving my stuff. I have to tell you, if I returned to my chair dripping wet and learned that the staff had removed my stuff there is going to be a problem.
I guess you could evaluate the amount of items on or under the chair. If you see a towel, t-shirt, flip-flops under the chair, you'd likely be correct in thinking the person is in the pool.
I've seen instances where the pool staff has cleared off chairs. But it appeared to me that they had observed the passengers, saw them stake a claim and then leave. Even then they waited a while before acting.
You can't run the pool deck like the Soup Nazi on Seinfeld. That is unrealistic.
Frankly I don't understand why people want to be so close to the pool if they are spending most of their time lounging. Other passengers are constantly filing by, bumping chairs, standing over you, stepping over you, you are more likely to be rousted every other minute by a bar waiter, etc.