Please help me understand computer hacking 'forensics'

After reading the post of the FB hack and how the company 'does not believe that user data was compromised' I thought to myself, 'How do they know that to be true?'

I'm sure this is not the right analogy (which is why I'm posting here) but if someone were to break into my house and take pictures of my belongings, how would I know? Likewise, if someone hacked FB data and simply took screenshots or some other kind of screen capture (or file copy), how would FB know?

Cheers.