In some cases you'll see that they leave your page immediately- too fast for a human to process that it's not what they want if they've actually clicked on an ad. Like, one or two seconds.

One or two seconds sounds pretty long to me. I think I close accidental ad clicks well under a second a lot of the time as I realize what I did without waiting for the site to load.

Yeah I do that too but I don't want to pay for those clicks either. I want genuine interested clicks. Google is set up to get me to pay for clicks, not to give me the right clicks.

Or at least it was when I used them. It's been a while because of that.

One or two seconds is not too quick for a human to reject your page after clicking on an ad.

It is totally too quick if they are looking for content. If they are looking for a car and click on a lumber store then I agree. But anything other than a clear mis-click and you can't absorb the page that fast.

I regularly click back from search results in under 2s if they're not what I was looking for; if I clicked on ads I would expect to do the same.

How the hell do you know? How do you know that the furniture store doesn't have what you are looking for in under 2 seconds? Seems impossible to me.

You can mostly guess from the layout.

Another tactic I'll use is actually parse thru apache log for adwords clicks looking for duplicate IP's. File a complaint with google to get a refund if I see patterns of abuse.