Of course, you can use an infrared thermometer to measure the temperature on both the back and the front (screen) of your iPhone (which is actually the most accurate method), or a thermal camera, but I'm going to suggest a third-party app here1.
Lirum Device Info Lite
This app gives you a lot of information about your device, including a lot of CPU-related information.
Lirum Device Info Lite is the most complete and elegant application to retrieve specifications of your device. You will be able to monitor its performance, watch real time CPU usage graphs, and discover a lot more about your iDevice. It's a system monitor and diagnostic tool for iPhone, iPad and iPod Touch.
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1You can ask me for more informations and guidance on using a infrared thermometer or a thermal camera in the comments.