How do I know if my company or I are on Google's blacklist???

If a site is banned from google it will not appear anywhere in the google index and the PR bar on the google toolbar will be grey (probably the only thing toolbar pr is good for).

I'm afraid that Google doesn't publish any information about how or who it 'blacklists'.

If you are still listed in the search, it is likely just a penalty.

How long do penalty's last?

Good question! Sites do come back to google after being banned. It takes time, depending on the type of spamming they did. My experience shows a site using hidden text comes back in google in 3 months time (after cleaning up there act).
Also it is good idea to mail google, apologizing for the mistake and requesting re-inclusion.

Believe me it works

Like so many other things, prevention is better than the cure. I heard one marketer say once that he never did get a particular site out of the sandpit. I think he'd been really bad.

As I recall this, most were being reinstated in 30-60 days from when the penalty was issued and Google notified. But don't quote me, I am known to be wrong a time or two.

It all depends on what the penalty is exactly. Penalties can last for months. They can last permanently if you don't correct the problem.

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Thanks for all the clarification on this penalty matter Jamison!