I haven't checked it and I am not sure how I could. I know that there is a long-term rise in tank temp. after hooking up this air pump, but it does not get out of safe range. The reason why I know it is happening is that I ran the tank for 2 weeks with no skimmer or air pump. During that period I would get about 1 degree rise during the day and now I get 2 to 2.5 degree rise.
Keep in mind two things: I am using a very high volume air pump and reducing the flow (back pressure) significantly so it is working very hard, and my water volume is about 1,100g. It takes a lot of heat to move that much water up a degree. I would assume that the temp. in the skimmer is probably about 1 or 2 degrees warmer than the general tank temp.