At present, Canadians abroad need to register. We have an international register of Canadians abroad, and they need to make an application. We maintain that register on an ongoing basis. When people apply, they need to prove their citizenship with a passport. We essentially require evidence of citizenship when you are abroad, for natural and obvious reasons.
In general, under the current rules, Canadians abroad can vote if they've been residing abroad for less than five years and intend to return to Canada. These are two of the basic criteria. There are other exceptions for military, for example, or foreign affairs.