How can I be tested for testicular cancer discreetly? We are ttc so don't want wife to know.

I don't know. What ttc means, or why you would hide a health concern from your spouse or partner, but you need a competent exam; likely next step is ultra sound, and if one or both are suspicious, markers (blood tests). No proof until biopsy, and most are done through the inguinal canal; not trans-scrotal.