With Male Bettas, yes. Definitely no more then one, as they will fight each other, sometimes to the death. Female Bettas can be kept together under certain circumstances, i.e at least 4, big enough tank, lots of hiding places etc. Males and females can not be kept together though.
Usually, males will have long, flowing fins, and females will have short ones. However, it depends on the tail type, but most likely you have a Veiltail. You can search on Google for pictures of male and female Bettas and you'll be able to tell the difference.
You are trying to sex the Betta in your tank, correct, not look into getting another one?