In some dog breeds you can tell the difference qiuckly and in others you can't such as:

The Ladbradore and the Golden retrever you can only tell the difference between its coat. the ladbradore has the shorter and Lighter coat and the Golden retrever has longer and darker coar.

the Shih Tzu and Malteze you can only tell the difference between them is by there noses. Th malteze has a longer nose and the Shih Tzu  has a shorter nose.