The Energy Research Centre (ERC) at VŠB-TU Ostrava has developed a methodology called the "semaphore method", which allows to determine what people heat their homes with by analysing a sample of ash taken from their boilers. This method is part of a broader project on air protection and proper home heating.
For details, see the video: Control of waste incineration in households
https://www.ceskatelevize.cz/porady/1096902795-studio-6/218411010101220/cast/664787/