On one of the yankie sites i can remeber some of the chaps taling about makes checks locally to be certain.
Maybe a bit extreme but it definetly is worth asking in as many ways as you can about the Girlfriends family etc.

I can remeber my Wife being worried to say she had a younger Brother and sister, why i will never quite work out.
She said i might not be so happy if i knew she had two siblings and its not like they cost us much in time or money.
For a while i was worried that they were possibly her kids but for a 24 year old to have a 18 year old daughter and a 14 year son would take some doing!!