Radio signal strength follows the inverse-square law with distance, so although a (cellular) phone uses relatively little power compared to other types of radio

Radio signal strength follows the inverse-square law with distance, so although a (cellular) phone uses relatively little power compared to other types of radio, your head would absorb more radiation in total with the phone so close. Layman's idea of mine: if you want to test specifically for microwave radiation, you could locate a cellular phone station (they are easy to find with various online maps, and are usually located on the roofs of buildings or lamp posts) and stand by it. I think that should be a considerably higher amount of radiation at any distance less than a few metres or so compared to holding your phone to your ear. The station will be transmitting constantly in any built-up area because cellular phones communicate with the station periodically even when there are no calls taking place.