'Sarah's Law' scheme covers all England and Wales
A scheme allowing parents to check if someone in contact with their child is a sex offender now extends to all 43 police forces in England and Wales.
The Metropolitan Police is the last force to join the scheme piloted and gradually rolled out since 2008.
Figures for 24 forces obtained by the BBC - covering 2008 to January 2011 - show that of 878 inquiries, 84 identified registered sex offenders.
The scheme's introduction has not led to vigilante attacks, police say.
Some critics of the Child Sex Offender Disclosure Scheme have expressed fears that paedophiles could be targeted in vigilante attacks.
Known as "Sarah's Law", the scheme was proposed after the murder of eight-year-old Sarah Payne by a convicted sex offender, Roy Whiting, in West Sussex in 2000.
The figures obtained under the Freedom of Information Act also show that, of the 878 enquiries to 24 police forces, in addition to the 84 which identified sex offenders there were 110 cases where further action was taken such as social services being alerted.
A father, who wished to remain anonymous, told BBC News that he had sought information from police about a man who toured village fetes and festivals and had befriended his son.
Police revealed the man had a conviction for sexually abusing a boy of a similar age.
"It obviously meant we couldn't trust this man under any circumstances... and we had to explain to an innocent 14-year-old... what exactly a sex offender was and what he might want," the father said.
"Without it (Sarah's Law), we would have no way of knowing this man was not to be trusted."
The man had offered to take the boy to festivals, and claimed he was a foster carer and teacher.
"It was as if he had thought in advance about the kind of things he would have to say," the father added.