How can you feel a baby guinea pig?

If you mean how can you hold it after it's born...:

It depends on the mother really. When mine were born, the mother had no problem in letting me hold her babies, but they needed feeding so i couldn't hold them for long. If the mother starts getting worried when you try and pick one up, then i suggest you wait a while until she's fine with it. Sometimes you could have to wait for a couple of weeks.