How to tell if a rim is 10ft tall?

At the park I go to there are 4 basketball courts but 2 seem higher than others people tell me the shorter one is 10 ft and the taller one is 11 ft. But I just don't believe it because I used to play in a 9ft rim and it feels like the one that they say is 10 ft. And at my schools gym which is 10 ft it feels like the one they say is 11 ft. so I would like to know how to tell the difference. I'm 5'9 so I'm not too short to notice the difference. I also don't have a high vertical so I can't touch rim on either hoop. There is also a little meter next to each pole that says they are all 10 ft, which is not true.