Here particularly - I too have serious doubts about calling him a "pedophile", but that he had a pretty good idea about the girl's age seems clear. Wikipedia has the following:

"According to Geimer, Polanski asked Geimer's mother if he could photograph the girl for the French edition of Vogue, which Polanski had been invited to guest-edit. Her mother allowed a private photo shoot. According to Geimer in a 2003 interview, "Everything was going fine; then he asked me to change, well, in front of him." She added, "It didn't feel right, and I didn't want to go back to the second shoot."

Geimer later agreed to a second session, which took place on March 10, 1977 at the Mulholland area home of actor Jack Nicholson in Los Angeles. "We did photos with me drinking champagne," Geimer says. "Toward the end it got a little scary, and I realized he had other intentions and I knew I was not where I should be. I just didn't quite know how to get myself out of there." She recalled in a 2003 interview that she began to feel uncomfortable after he asked her to lie down on a bed, and how she attempted to resist. "I said, 'No, no. I don't want to go in there. No, I don't want to do this. No!', and then I didn't know what else to do," she stated, adding: "We were alone and I didn’t know what else would happen if I made a scene. So I was just scared, and after giving some resistance, I figured well, I guess I’ll get to come home after this".

Geimer testified that Polanski gave her a combination of champagne and quaaludes, a sedative drug, and "despite her protests, he performed oral sex, intercourse and sodomy on her", each time after being told 'no' and being asked to stop." "

Now, there are some parts in there that are he said/she said, and that Geimer might be exaggerating on - though judging on her recent comments, she doesn't seem the type to make things up or exaggerate very much. The article doesn't say which if any parts of that story Polanski disputes, though surely there must be some if he was offered that plea bargain... the accusations as described there are very serious, I can't imagine he would've gotten off lightly for those. But the part about him having talked to her mother and the two sessions deal seem like they would be very easy to check, and hence rather unlikely to be untrue. Of course talking to her mother doesn't automatically mean he knows her age, but at least he must have known she was a minor, and not just technically.