I carry a thermometer with me when I fish, but most of the time I don't feel that I need to use it. However, this summer I have been using it faithfully, as we are having a record drought in the west. The drought is causing not only very low water levels, but also record water high temperatures.
Most reputable sources say that water temperatures above 65 degree F (18C) will greatly increase trout death. So, I personally practice this rule: if I find the water temperature to be above 65 degrees F, then I don't fish that water. The only way you will be able to tell what the water temperature is, is by using a thermometer. Don't rely on your hand to tell you what the temp is, please use a thermometer.
Water temperatures here in the west are so bad, that Yellowstone National Park has put a fishing ban on streams and rivers parkwide after 2PM. This rule is in affect until further notice by the park service. Yellowstone has some of the most pristine cold water riverine habitat in the lower 48 states, so if they are banning fishing, then all the rest of us ought to be paying attention to our waters. If we fish the high elevations of Colorado, the lower elevations of Appalachia, or even previously safe tailwaters, let's all be good trout stewards and check the water temperature first. Don't assume your waters are safe! Please check the temp.
We live in crazy times!