If you have the site registered in Google Webmaster Tools, you get a message within the GWT system about the penalty, and an area where you can appeal it. You also get warned about malware infections, and I think now old installs of Wordpress.

I had a client get caught in one a while ago, the notification isn't perfect and at the time (I don't know about now) they didn't e-mail the account holder as well, which I thought was a bit daft as they have your e-mail as it's your login.

For the client, when I cleaned up their site and wrote an appeal saying what had been done, the Webspam team were very quick at processing our appeal and restoring the site. It wasn't a buying links penalty though, it was about stuff they had on the site from some dodgy advice they'd had a few years before that I'd been trying to get them to remove, so it was easy to show they'd learnt their lesson in the appeal.