How does Background Investigation(BI) work?

[metadreamerboy]

How does BI work? What specific questions are usually asked from previous employers? How would a job applicant know that his/her previous employer is badmouthing him or her?

[bishop]

The bigger companies usually employ a vendor to do the background/reference check for them.

How would one know if a previous employer is badmouthing him/her? Simple. He/she won't know.

[metadreamerboy]

does anyone know what questions are usually asked? what are the legal paremeters in conducting a BI?