“is it possible to marry someone who is not your bashert? if someone gets married can we say with certainty that that person was their bashert? “
The answer is yes, one can marry someone who is not their bashert, even if that person was destined for someone else. Then the answer to the second question is obvious; one cannot say with certainty that a given marriage was bashert.
This answer is from Sefer Chassidim (Rav Margoilis edition), se’if 321 ,near the end of the footnote .