This article is from the Getting A Cat FAQ, posted to rec.pets.cats newsgroup. Maintained by Cindy Tittle Moore with numerous contributions by others.
Have your new cat tested for exposure to Feline Leukemia Virus (FeLV). If the cat is positive, you will need to keep the cat indoors, separate from all other cats, or you run the risk of infecting other cats. See the Feline Leukemia FAQ for more information.
Other common tests are for Feline Immunodeficiency Virus (FIV) and Feline Infectious Anemia (FIA).
It is not possible to test directly for the deadly disease Feline Infectious Peritonitis (FIP). There is a test sometimes known as an "FIP Test," but this test actually does not test for FIP or for FIP virus. It tests for exposure to viruses in the coronavirus family (FIP is one of many coronaviruses). If you do decide to use the "FIP test," be aware that its results are very difficult to interpret correctly. Perfectly healthy cats often test positive on this test, even if they have never been exposed to FIP. If your vet believes that an otherwise healthy cat has FIP because of a positive test result, you may want to seek a second opinion.