An easy example of behaviour test

I'm new to writing behaviour tests instead of "classical" unit test. I've read some articles about this but there is only abstract examples and I don't know how adapt it for my need. It would be great if you give me example of test for this case:

We have Uploader class with method Upload, which calls private CreateRequest and then after some time should be async call of HandleResponse. How can I test this behaviour?

You can use pseudo code if you want to or c# NUnit code (would be really useful for me).