When you've found said non-existent uneroded stone then you would look minutely for tooling marks. The alternative approach would be to find some putative groove carvings in a dense rock - the Devil's Arrow eg - and then to measure the width of the grooves in Megalithic inches. The answer should be an integral even number. But I must admit I haven't tried that approach with this stone http://www.themodernantiquarian.com/post/5654 yet. It's Millstone Grit so relatively fresh. As well as deep grooves to the left, there's seems to be an engraved figure to the right.