See photo
This sheet was a free sample I got from LCR Hallcrest - liquid-crystals.com They are in Glenview, IL, here in the USA. They also have a UK office.
They have a lot of different applications that use them. They respond to temperature changes almost instantly.
It seems to me that for electronics applications, cheap and reusable liquid crystal sheets likely work almost as well as an expensive thermal camera in many situations.
You can also use it for finding leaks around windows.