Several questions about file upload

[eluser]bewhite[/eluser]
I have tried to use standard upload library and several tings are unclear for me:
1)I can find all file attributes in the $this->upload->data();, but where can I find the uploaded file content? Is it in the $this->upload?
2)How can I determine if file was uploaded or not?

Sorry, for these question but I'm still noob at CI