You should have piped up at the time. That being said make sure it is a wart, google it, before you start thowing accusations about.
Be sure its not just a spot or a skin tag or something like that.
Pretend to be a new punter, make a discrete enquiry about STI checks. If they get angry, then ring the alarm bells.
Most girls in my usual area, Belfast, are happy to talk about their STI checks, if you talk about yours.