They test all women for rubella immunity at the begining of their pregnancy, it is one of the million tests they run. The weird thing is I come up immune to rubella and not measels and mumps. You would think if it was from the shot I would still be immune to all three. The Dr's who work with my midwife wanted me to get vaxed with MMR after my son was born but I declined. My midwife figures I probably had Rubella at some point either after the false immunity from the vax wore off, or because the vax never worked. But the disease is so mild in so many people, and I was vaxed so no one ever noticed it and figured out what it was.
All that to say it is possible that you are immune because you caught it and don't remember catching it. Or you could still be immune from the vax as a child. If you and I are around the same age you would have been vaxed for the last time sometime around 12-15yrs of age. I dont' know what the mandate maybe before that, and there is probably more now.
I do know alot of women are being encouraged to be vaxed right after delivery because it is discovered when they get pregnant that they are no longer immune to rubella. So much for mandatory vax's protecting against Congenital rubella.

:
ETA: should you discover you are no longer immune to rubella you have two choices, take the vax (which I wouldn't do) or try to avoid any children who may have it or who have been recently vaxed for it. The vax is live and sheds and so recently vaxed kids are probably the most dangerous to pregnant women. It is definately better to get it naturally and have natural immunity which is far less likely to fade (though I have read that natural immunity doesn't always show up on titre checks).