say: oi. is you got the clap?.
"Have you ever had an STD?" If they don't answer or get offended then they definitely aren't worth bothering with.
I say "what the **** is that" ?
Seriously it's a tricky one but you must be sure before you enter into a sexual relationship, best thing is to be up front and just say "Have you been checked out for any STD's lately" especially if its a new relationship, qualify this by saying, you never know if your previous partner had something so you are not accusing them of sleeping around. That should do it. :-)