Radio energy drops off pretty quickly. If the question you hoped to answer was does RF at any level higher than the background from the Sun + cosmic background cause cancer then that would be difficult, but if the question was does holding a smartphone cause cancer it would be relatively easy. It would also be easy to give people a device that either transmitted rf or didn't with no outside indication. Also really easy to do this in mice or the like. So far all indicator are that for the sort of frequencies we use the only impacts are heating