People that haven't seen it are the ones who mostly do not accept that it is happening. E.g., in the continental USA, changes have been pretty minor so far. A 1 to 1.5C increase in average temperature - maybe, depending on where you are. Hardly enough to be noticed by the casual observer.
However, in areas where the changes have been more, e.g., northern Canada, and Alaska, the changes have been pretty dramatic, and, easy to see, even for the casual observer. Which is why people who live where I do (Yukon Territory) get somewhat upset with the people who say it isn't happening, or, who try to bring politics into it (your politician says it is happening, mine says it isn't, therefore it isn't).