"My mom's a sex therapist, so she's great at talking about relationships," the O.C. star, 25, tells the new InStyle spinoff Your Look in its spring issue.
"She knew the day after I lost my virginity. Oh, yeah. She starting talking to me about sex. She said, 'It happened last night, didn't it?' I was like, 'How did you do that?!' She did the whole mom thing, of course: Use condoms, etc. Weird, but helpful."
But Bilson, whose three-year relationship with costar Adam Brody, 27, ended in December, has learned a few things about guys on her own.
For one thing, she says: "They have different body parts."
She also suggests a trick for telling whether he's the one: "If you're scared he's going to try and kiss you, it's probably not right," she says. "If you really want him to kiss you, he's the guy."
She will share one secret though – how to fake a killer figure. Bilson, who insists, "I don't feel any pressure to be thin. I like to eat," tells the magazine: "On the red carpet, there's definitely a stance. My chin goes higher, and I automatically put one hand on my hip to give my body shape."