Raf was diagnosed in October last year, aged 9. Vet didn't suspect Cushings and only tested as an afterthought because the test was free in October. Raf's symptoms were slight weight loss - when I was on holiday in September though, which was worrying because he usually puts weight on when he has his two week summer break - his coat looked dull and although he seemed fine in himself, flying about the field etc, ridden he just felt too 'good' - he did everything asked but there was absolutely no spark there at all. He's never had problems with his feet and the vet said not to change his diet because the diet is to help the feet, not the Cushings itself. His weight loss was most pronounced on his top line, but he got a rounder belly, which I later learned is typical Cushings. The symptoms were so mild though that unless you knew him and knew what he should be like you wouldn't have guessed he was ill - in fact plenty of people told me I was worrying about nothing.
He had a bad reaction to the Prascend, even though he was only on half a tablet a day, and had to come off them and restart on a 1/4 after a couple of weeks' break, it was still several weeks before he came out of 'the veil'. He's now back on half a tablet a day and pretty much back to normal, although he has some days where he's full of life and others when he's much quieter- not sure whether that's the Cushings, the mild arthritis he was diagnosed with later (another story) or just him. The vet said the tablets were about £1 each, but I think they are actually a little bit more from the vet, although the insurance is paying at the moment so I haven't researched to find the cheapest option.
Have you had the test yet? What makes your vet think it's worth testing for? Hope you get sorted soon.