Female guinea pigs have a sort of shaped vagina.
Males have a bit of a y but with a dot at the top.
Here, I don't know if I'm allowed to post links but here is a site that will help you...http//www.cavyspirit.com/sexing.htm
Where it says to extrude the penis...well that's not very easy to do.
The best way, if those pictures don't help you, is to take your guinea pig to a vet and get it sexed.