You can tell it is hot from the attitude of the dragonfly. The hotter it is the higher they raise their bodies to the vertical. This enables them to regulate the amount of their body that is presented to the sun.
It is just something that you learn if you study this order of insects. Birds act in a different manner. They spread their wings and lift them away from their body and pant. Dogs pant and so it goes on.
There are some good books that deal with insect behaviour. One of the very best is The NEW Encyclopedia of INSECTS and their allies. It is published by Oxford University Press and Edited by Christopher O'Toole.
and is VERY VERY GOOD and written so that those who like their text in non-technical languge cam follow it with ease. Try Amazon for the book and you will get hours of pleasure from the text and the superb photographs. Chris. O'Toole is from the Hope Entomological Collection at my old university. ie Oxford.
This book comes highly recommended by myself and I am not easily pleased when it comes to books on natural history.
Thanks for the info. I will order it. It is satisfying to me to understand what I am taking pictures of. I appreciate the species more and can present it appropriately. Knowledge is a great thing and learning is easy when you are enjoying the process.