You don't know, you hardly ever know anything. You evaluate the evidence (as laid out by the GP) and conclude that it's most likely the case.

This is really the approach that most conspiracy theories take. The fact that we don't know something to be true (but it's very likely to be) is used as an implication that it's probably false.

If that possibility seriously concerns you, follow it up by physical destruction of the drive. Magnets strong enough to actually damage the data aren't a good cost/benefit from what I gather (too strong, too dangerous, too much hassle), so either shred or melt the drive platters.

I'm generally satisfied with df if=/dev/zero of=/dev/sda because I'm not actually dealing with anything that sensitive, but if you are, just take appropriate steps. Defense in depth.

I think that incinerating the disk should erase the data. Magnetic material become normal materials after some temperature http://en.wikipedia.org/wiki/Curie_temperature . Usually it is no more than 1000 C/ 1800 F, so a good fire should be enough.

Warning: I don't know if the fumes are toxic.

If you're that worried about it, there's always thermite.

Well, you don't know. It all depends on what kind of data was on the target drive.

If it's schoolwork or unimportant business data, a format is good enough. If we're talking CC and SSN numbers, the ATA command with writing 0's is good. If it's sensitive, a sledgehammer and an anvil is suitable.

I inherited a junk 1G hard drive, and just 'filed it away' in my junk drawer. After years, I actually checked the contents. Lo and behold: a flat database full of medical data, including SSN's and other revealing data. It was only 1G so it got the sledge (I use a small section of railroad tie instead of a blacksmiths anvil).