How do you know that you have used aseptic technique correctly and have achieved a pure culture?

Answer 1

You should only see one type of colony growing on the plate. If there are more than one different appearing colonies, then it is not a pure culture.
eg. white colonies in a culture of golden staff would indicate an impure culture
There is no way to tell properly until the plate is incubated but if the culture is not pure, it is easy to subculture again to obtain a pure culture. Can get frustrating at times, but is achievable :)