This article is from the Feline Leukemia (Cats) FAQ, posted to rec.pets.cats newsgroup. Maintained by Erin Miller with numerous contributions by others.
Unfortunately, since FeLV is a retrovirus that attacks your cat's immune system, your cat can become ill from many things as a result. This makes looking for a 'sure sign' very difficult. Often the immune system is weak so your cat will become chronically infected with certain conditions such as stomatitis, gingivitis, oral ulcers, abscesses and non-healing wounds of the skin, upper respiratory infections or FIP. Some cats whose digestive tracts are affected have been described as staring at their food bowl seemingly unable to remember how to eat, or their breathing will be very difficult and loud. Basically, whenever your cat shows chronic, peculiar and/or unhealthy behavior, take it to a vet to be examined.