Double check. Put your index finger crosswise on the abdomen, right at the top of, and very close to, the genitals. Press down, and rub your finger back and forth across the abdomen. A guinea pig penis has a bone in it, called a baculum, and you can feel it when you're rubbing back and forth.
If you also press down and toward the feet, you may be able to extrude a penis. No penis, no penis bone, it's a girl.