Not all cats bury their waste, although I used to think so, it seems increasing numbers don't - so the only way to say a fox is involved is to see or photograph one in the act (or have an analysis of the stuff carried out). Unfortunately, cat owners are reluctant to acknowledge the problem because they don't suffer it while their animals dump on the neighbours. There is a very effective fitting available which goes onto a garden hose and actually targets/tracks the cat as it moves about the garden. I saw something about this not long ago, sold on Amazon amongst other places, originates in the USA if I remember correctly. Extremely amusing video somewhere online - scares the bejeezus out of the critters (much like placing a cucumber near them when not looking - another set of very funny videos) and they quickly learn to avoid the place. In fact, I think some of this may have cropped up on AB not long ago.