Pregnancy. Having tubal sterilization is not like tying your laces where they can be "untied". Tubal sterilization usually is performed by damaging a portion of the tube with electrocoagulation, placement of clip/ring, or removal of portion of the tube. When treatement failure occurs (~2% over 10-year period), it usually occurs as fistulas or small openings occur, not because the preocedure is reversed.
X Ray. You can have an X-ray performed (hsg - hysterosalpingogram) to confirm if the tubes are still blocked. Unfortunately some women also find out when they become pregnant unexpectedly.