>>And what are the tell-tale signs that it's a cup, formed by man, or a natural process of weathering?!

ahhhhh... the million-dollar rock art question ;)

I think if it's got a level base inside the shape then it is more likely to be man-made. Weathering tends to be more - er - spherical (?) but then so are some classic small cups...

When up ont moors I took a pic of a basin which had clearly been produced by erosion - because the flakes of granite / crystals of quartz were still present in the bottom. All sitting in a nice pool of water. It was clear this had been created from freeze/thaw natural processes...