Is there a way to detect which side of the vas deferens is blocked?

This is most often: done by palpation...feeling during a physical examination. If there is an obstruction, the epididymis and vas may feel dilated and hardened; normally it is soft. If more definitive diagnosis needed, an ultrasound may be helpful. Ultimately, dye CAN be injected into the testicle which can show a blockage in the vas. Your urologist will be able to advise and carry out this test if needed. Good luck!