My 3 year child has just accused my fianc�e of sexually molesting her. We just had DCF here today?

she was questioned about a non-sexual abuse investigation that had nothing to do with Tom or myself. She admitted this to me by herself much later after Department of Children and Families completed the home visit and left. My child has an issue with lies and story telling so it is hard for me to believe her. She is 3 years old and very imaginative. I know this sounds like denial and im aware that could be my issue.... but is there any concrete way to know that she is telling the truth?? Doe they give lie detector test to children? How can I be sure shes not making it up?She told me that she told the investigator the same thing and if she had admitted this to DCF wouldn't they have removed her immediately? Our family has been going through some very stressful things lately and this is the most awful thing that could have ever happened i just want to know the truth... how do i get it and am i a bad person for not believing her?