Naturally you need a male and a female fish. First make sure you are looking at an adult fish. It is very difficult if not impossible to tell the sexes apart in juvenile fish. Generally all the corys sold at aquarium stores are young ones, so usually corys have to be kept for a year before they reach adulthood in our tanks. In adult fish females have deeper and wider body whereas the males have narrower and longer bodies.
Females usually grow larger as well.
With a few species of corys the sexes can be identified by looking at the pelvic fins alone. Where the male has pointed pelvic fin and the female a rounded one. Corydoras cats spawn in groups. Usually 2 males per female is the formula given by many people who bred them. Having said that a single pair is totally capable of completing a successful spawn and the ratio is not that crutial though more males than females gives you a better chance.