There are no obvious signs of FIV so the only way to know is to do a blood test. The most common screening test is an ELISA test (often called a SNAP test) done by your veterinarian, which looks for antibodies to FIV. An antibody is a protein made by the cat in response to FIV infection. A cat can test positive as soon as two to four weeks after exposure, but it can take up to eight weeks.
Kittens under six months of age may test positive after having received antibodies from their mothers, either in utero or via milk. It can take up to six months for these antibodies to go away. Thus, it is a good idea to retest a kitten who tests positive after he or she has reached six months of age.