I think we've only got one pipe, at least up until the prostate, where the rest of the stuff gets mixed in.
The suspicious tissue where the inflection can start and the allery chain may be activated is the mucosal epithelium, which is a lymphatic tissue which lines the urinary tract.
These tissues, line much of the male reproductive and urinary tract. Starting from parts of the testicles through the epididymus, vas, prostate and urethra.
Whether injecting in the urethra is a proof positive test, I would doubt, but it's worth trying. As a matter of fact, there are a few looking into it.
For my money, the epididymus could be a very suspicious possiblilty, as it is a very delicate tissue of very thin condiuts meters in length... I don't remember how many. However we can't inject semen there other than having an orgasm....