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To check if some of your babies are males (boars), press gently just above the genital area. If it's a male, you should be able to make the penis slowly come out, due to your gentle pressing. However, don't be fooled if you don't see a penis present; sometimes the penis can be tucked away and out of sight, in folds of skin (usually in older males, so with the babies, it should be easier), making it look completely like a female. You should try to get the penis to ease out, just to be sure you have a male. In addition, younger males will have a donut-like shape around their anus, where their testicles are located.
If you're still not sure of their genders, bring your guinea pigs to a vet who is experienced in treating them; don't bring them to Petco or another pet store, as the clerks may not give you the correct information you need. Only a vet who's used to working with guinea pigs will be able to provide the most accurate information on which ones are female and which ones are male.