It's not always that obvious MiceElf. Much sexual abuse happens within the home, so is largely hidden. Stereotypical paedophiles might be easy to spot, but the young man you describe not so much. Here's one of many links that might fit the bill in terms of your query.
crime.about.com/od/sex/p/pedophile.htm
It's the powerful figures of authority that manage to disguise their abuse and keep ther victims silent. Think of any person who has power over a potential victim, and there should be safeguards in place eg a nurse to accompany a doctor doing certain physical examinations, teachers beng vetted and CRB checked, as are residential workers who go in bedrooms.
Women who abuse are significantly more difficult to identify when abuse is disguised within 'caring behaviour' like bathing, nappy changing, bedtime routines.
Horders of abusive images might not be noticed, but anyone who is spending more and more time being secretive about what's on their PC, invites children into their room to play on the computer without adult supervision, appears to be overly familiar with families, keen to 'rescue' eg 'go on, don't worry, I'll look after the children for a few minutes' when mum is single, may be vulnerable. I could go on and on, but you get the gist, I'm sure.
The other thing to look out for is changes in behaviour in children who can't or won't tell. NSPCC provides checklists on websites, as do many local social services departments on their Safeguarding Children pages.