Does my cat still need a heartworm test if they are on prevention?

No, we typically don't test cats. Especially as a kitten, we just start the prevention. The only reason that I would recommend a test is if they're having symptoms. If they're having breathing problems or GI upset, then we might want to test for heartworm. But it's not something that we would routinely test every year, at least not in our area in California.