antibodies after 1st vaccination

A couple years ago i started the series of vaccinations for hep B, but only ended up getting the first one.  6 months later i had a physical and explained what happened to the doctor, and they tested my blood to see if i had developed the anitbodies already. well i had, but i was told this was rare after only one vaccination.  i recently saw another doctor and was told that it was more likely that i was exposed to hep B at some point and that is why i developed the anitbodies.  is it possible to know which actually happened? and if i was exposed to the virus, is it possible to find out whetehr it was before that vaccination or after? if you could get back to me soon, i would greatly appreciate it.