The skin never really appears smooth, but I guess I know what you're talking about.
Is there a big, thick fleshy pad on the frog's back? This is where the eggs would be if they were still present. Sometimes you can still see them through little window like holes in the skin.

I only have a single pipa, and don't have any first hand experience breeding them. Most of what I know about them is through researching them in books and online