Petricola stay small by syno standards and aren't the easiest to sex. The male has two holes (To keep it as simple as pos.)but the one closest to his tail will look slightly elongated whereas the females will not. And yes, the girls tend to be plumper around the belly as they mature. I have a girl but sadly no pics of her so I can't show you. If yours swims along the glass with his/her belly against it as mine do you can see their nether regions pretty well. Here is an example of a male and a female, although the female is obviously carrying alot of eggs in this pic. Some info here may be of help too.
Hugs,
P.