C. diff is diagnosed through a stool culture. Most GI's do these routinely when they first see you - you know, when you have to take in four or five little cups of poo? At least, both of my GI's have. After that, testing is done if the specific symptoms of this infection are present. It is treatable with a few antibiotics, but can it can be a long and difficult course to get rid of it.
C. diff is also the primary concern with antibiotic therapy, as it is opportunistic and will inhabit a sterile colon and increase rapidly. This is why big doses of probiotics are important when on antibiotics.