Although endoscopy is not routinely recommended for the diagnosis or management of C difficile infection (CDI), this procedure may demonstrate the presence of raised, yellowish white, 2- to 10-mm plaques overlying an erythematous, edematous mucosa. These plaques are termed pseudomembranes (see the following images).
http://emedicine.medscape.com/article/186458-workup
The following are stool assays for C difficile, from the most sensitive to the least sensitive: Stool culture is the most sensitive (sensitivity 90-100% and specificity 84-100%), but results are slow and may lead to delay in diagnosis if used alone.
http://emedicine.medscape.com/article/186458-workup
Cell cytotoxicity test is a tissue culture to detect the C. difficile toxin. It is a test that looks for the effects of the cytotoxin (cytotoxicity) on human cells grown in culture. It is a more sensitive method to detect toxin, but it requires 24 to 48 hours to get the test result.
https://labtestsonline.org/understanding/analytes/cdiff/tab/all/
While most C. difficile colitis in the US is caused by antibiotics, C. difficile colitis also can occur in patients without exposure to antibiotics. For example, patients with ulcerative colitis and Crohn's disease have been known to develop C. difficile colitis without exposure to antibiotics.
http://www.medicinenet.com/clostridium_difficile_colitis/page4.htm
Although the antibiotic clindamycin (Cleocin) has been widely recognized as causing C. difficile colitis, many commonly prescribed antibiotics also cause colitis. 1  cephalosporins (such as cephalexin [Keflex]). 2  quinolones such as ciprofloxacin (Cipro).  aminoglycosides (such as gentamicin [Garamycin]).
http://www.medicinenet.com/clostridium_difficile_colitis/page4.htm
C. difficile infection is the most common cause of diarrhea in people who develop diarrheal symptoms while hospitalized. C. difficile toxin is detected in the stools of up to 20-30% of those with antibiotic-associated diarrhea and greater than 95% of those with pseudomembranous colitis.
https://labtestsonline.org/understanding/analytes/cdiff/tab/all/
In fact, metronidazole and vancomycin are two antibiotics that are used for treating C. difficile colitis; however, there are rare reports of C. difficile colitis occurring several days after stopping metronidazole.
http://www.medicinenet.com/clostridium_difficile_colitis/page4.htm
The Test Sample. Clostridium difficile (C. difficile) is a type of bacteria that is associated with diarrhea resulting from antibiotic use. C. difficile testing and C. difficile toxin tests identify the presence of these bacteria and detect the toxins produced by them.
https://labtestsonline.org/understanding/analytes/cdiff/tab/all/
Since many antibiotics can cause C. difficile infection, all antibiotics should be used prudently. Self-administration or using antibiotics without an accurate diagnosis or a proper reason should be discouraged.
http://www.medicinenet.com/clostridium_difficile_colitis/page4.htm
Antibiotics can sometimes cause diarrhea that is not due to C. difficile infection. The reason for the diarrhea is not clear. The practical implication is that not all diarrhea associated with antibiotics should be considered to be due to C. difficile and treated as such.
http://www.medicinenet.com/clostridium_difficile_colitis/page4.htm
what tests are done to diagnose c diff?
A cell cytotoxicity test is done to diagnose Clostridium difficile.  Cell cytotoxicity test is done to diagnose Clostridium Difficile.