How do you tell the gender of a pirahna?

No. There are no notable differences from Female and a Male Piranha. None that we have discovered anyways.
In the several species that are all commonly called "Piranha" there appears to be no sexual dimorphism. About the only way to tell the difference is the females tend to be a bit chubbier than the males.
You cant usually tell the difference between male and female piranhas.

There are no notable differences. So if you are looking to breed you will not have very much luck with just 1-2 piranhas.