There are antibody tests to show if you've ever had it, and approximately how intense/active the infection is. Mild latent and recent past infection is hard to distinguish apart from an antibody test alone. Decreasing antibody concentration over time = fighting it off successfully, or past infection fading away (probably). Antibody tests like that aren't terribly expensive. (Well under $100 here in Canada if uninsured.)

Direct testing for the parasite's DNA in the cerebral-spinal fluid (yeah, a spinal tap sorry) is sensitive, too. Attempting to detect it elsewhere (blood, etc.) works but isn't as reliable. This is more expensive but sometimes used when unsure. Otherwise, in combination with that, it's diagnosed symptomatically. It shows up on a brain scan if the lesions get big enough, not to be alarmist.