unless we are suddenly going to find patrols of rangers ,law enforcement,or the CAA ,FAA,going out with specialised equipment to determine the height of our UAVs during a flight then all we can do is use some common sense and do our best to conform to the 400ft rule, not an issue,if the area you are flying in is relatively flat, a somewhat different matter in mountainous terrain where i fly in Wales i probably inadvertantly fly above 400ft AGL without even realising it at the time, because of the terrain i am flying in,its all a matter of perspective its much easier to determine your height from your home point if you keep in VLOS which for me is around 1,200ft with my old eyes