Oh God, what an awkward question. Part offensive, part paranoid, and altogether unavoidable.
I guess the general rule is that if you're not coming in contact with someone's junk, it's none of your business. And trust me, I don't want to be the girl to freeze before locking lips and ask about any history of cold sores, but I don't want to be at risk.
XOJane's Ask Emily answered a question about how to approach the subject, and frankly, I hate the response. She wrote that the boy asking sounded like he was "in danger of becoming one of those guys whose STD paranoia sucks all the fun out of casual sex." She goes on to recount her experience trying to suck off a guy after a first date, when her partner stopped her and screamed "Don't put your mouth on it!" First of all, good for that guy. I wouldn't trust unprotected oral on a first date either. And as much as it probably offended her, he had every right to stop her. Sex carries risk. And people are allowed to protect themselves.
At the end of the day though, everybody can lie. Whether or not they mean to. Most sexually active people will get at least one type of HPV in their lives, and most of them don't know they're infected. Whether or not you ask, be prepared either way, and know how to protect yourself.