Someone asked me this question today and I didn't really have any concrete evidence to back myself up. I want to know how DP members determine when a site has a penalty...I'm not talking about a small duplicate content filter. I'm talking about serious penalties. Spam, linking to bad neighbourhoods etc.
If you go to iWEBTOOL they have a tool for checking whether a particular domain is blacklisted by Google. An 'established' site with lots of content that displays PR0 is also likely to have been penalised. Editted to add: http://www.iwebtool.com/google_banned
Get the site verified in google webmaster tools. If there is a penalty in effect, they will tell you in there. You are on the right track though by separating filters and penalties. However if you are carrying out any sort of black or grey hat SEO then you just have to be prepared to accept the death of the site.
I think I was linking to two sites that had penalties...I quickly removed their links. Last week one of them had a PR of 5 today the Pr is 0....the other site was justfreestuff.com, they are losing index pages and their PR is also down to 0. My site also fell from number 1 to number 121 for my keyword.
An established site with 0 toolbar pagerank is a telltale sign of a serious penalty.. being dropped entirely from the serps is another sign.
Old Welsh Guy knows his stuff... but this one, hmm... Would you mind elabrating on this a bit? How and where did you find this info on the webmaster tools? Please tell more ps: Bans are easy of course, but we're talking a penalty here.
Oseymour, thanks for bringing this up, and CountryBoy - thanks for the link! I have 2 sites that are very well indexed & not new but still, both stay on PR 0. So I just checked both and they do not appear to be penalized whatsoever. No idea how come or what might be up.
I think my site has a penalty because I found out I was linking to 2 shady sites that suddenly lost their Toolbar PR and rankings, you can't even find them if you search by name