ask them?
you cant find out a persons record any other way unless its been made public knowledge and you can find it in court records or newspaper articles
If he is a new boyfriend you may be able to find out. I don't know if this proposal came into force.
"Under the proposals members of the public will be able to seek details about someone with whom they have a personal relationship and who has unsupervised access to their children. Women will be able to request the information from the police about new boyfriends as soon as the relationship starts. " http://business.timesonline.co.uk/tol/business /law/article1929197.ece
hc - This is being trialled by a small number of police forces - from memory Cambridgeshire and Warwickshire are involved Im sure their websites will have full details.
I recently found out my friend was a sex offender and i searched the internet of ways of clarifying this but in the UK you cant, i eventually put his name in to www.123people.com and it brought up all news paper articles related and confirmed it that way.