For a more interesting version, go find your site in the Google Directory and see where it ranks compared to related sites with a little more subtlety.

There's no special public Google URL to check the PR.
The PR is checked by doing a special call including a key and some encoding to certain Google address.

So, unless you want to create your own tool, the only thing to do is to use those tools already done by others.

I found some scripts that would do this. One used a Google URL which now returns a 403 'forbidden' response.

Do you know if those scripts/sites which _do_ work are legitimate, and Google doesn't mind them. Or are they getting in 'under the radar'?

Could one get one's own tool developed, only to see it banned by Google if thousands of people started using it?

As fas as you don't abuse with hundreds of request by second, I think you can get unnoticed.

If you put out your own toolbar, and thousands were using it, that could add up to 100's of requests per second i.e. a drain on resources, which Google wasn't getting anything out of.