I have a schoal of 10 penguin tetra. The largest of these is significantly fatter (not just longer) around the belly than any of the others.
Is there any way of telling if it is a pregnant female, and should I do anything special if it is?

Tetras are egglayers and do not get pregnant.

Was this fish the biggest before you got them? Cause if it is, it is very likely the dominant member of the shoal, the "Big Kahuna" who can pick on anybody he/she wants without retaliation. This is called a "pecking order" first observed in chickens, where the most dominant member could peck anyone below him/her, and the second most dominant animal could peck anyone below him/her, and so on until there is just the littlest guy at the end who doesn't get to peck anyone. In higher species of animals such as Lions, an animal that is not dominant will sometimes challenge the "Big Kahuna" for dominance of the pride/herd/flock/ect. It is these large ones that also get the most food out of the shoal, so your large Penguin Tetra is probably just a big, dominant pig, and this "pecking order" is often the explaination for why a new fish gets picked on. But if your largest Penguin Tetra dies then the next largest fish will probably replace her, so it works in a cycle.

-Lynden

It is possible that it is full of eggs.. I had a fat one years ago that expelled a bunch of eggs.