- And got the T-shirt
A vet probably wouldn't charge you much to tell you what the sex is.
Our guinea pig savvy vet sexed our babies at no charge. Find a good guinea pig savvy vet, tell him/her your situation. If they're smart they'll sex your pig for free because they know more business is coming their way. Heck, they'll probably sex your babies (when they come) for free too.
Because charging to sex several pigs is very short-sighted when you have a possible neutering, pregnancy complications, or even just regular run-of-the-mill health needs of several growing pigs of various ages coming down the pike. A smart vet will sex your pig for free, hoping you'll return when you REALLY need them.
Even if they do charge you for a full exam, they will give you a full exam, which your pig might need as a baseline.
Btw, when you are looking for suitable homes, NEVER say "free to good home." Reptile owners are always looking for "free to good home" rodents as free food. Many babies -- and even adults -- who are advertised that way end up as snake food. I've known of snake owners who will even lie to the person with the rodents and assure them they want them as pets, even showing photos of a cage set up or the inside of their home or whatever. Charging a fee large enough to discourage this (at least charge more than buying rats meant for snake food) is what you will need to do to ensure your cutie pie babies don't end up that way.
I had planned to sell my babies back to the pet store. When the time came, I couldn't bear to do it, since I knew they didn't screen who they went to. I wasn't afraid of them ending up as snake food from a pet store, but I was fearful that they might end up mistreated or abandoned.
All that to say that you should hold off on plans to get another pig companion for your lone pig now until you KNOW s/he won't be compatible with any babies that are on the way.
In any case, he wasn't obviously male until his testicles dropped into a sac. He was rumblestrutting a bit before that but we had no idea that was a warning sign as we were newbies still. Even after his sac dropped I stupidly thought "she" just had "swollen" genitalia.
He passed the ambiguous genitalia trait onto two of his 7 offspring. They were almost impossible to sex when very young. The only way we knew for sure was when the actual guinea pig savvy vet sexed them for us, and it was even difficult for him to do for the girl. Her outer parts looked like a classic male, but there was no penis bone. He tried and tried to extrude a non-existent penis for longer than I would have wanted if I had been that poor little pig, lol. The boy actually did look just like a girl, but oops, there was a penis in there! The other babies were easier to sex.
All that to say if you have one with truly ambiguous genitalia, you absolutely need a guinea pig savvy vet to make sure.
I have one of these. I love it for reading tiny print and it has come in handy for other things (as I mentioned, 2X magnification).