Firstly, apologies if this has been asked a million times but I couldn't find anything? How do you find out if you have a Google penalty? Is it through Google Webmaster Tools? Thanks.
First use the site: operator to make sure pages from your site are in the index. If not, you've been banned - a major penalty requiring a thorough clean-up and a Reconsideration Request. Second, search on your domain name - "yoursite.com". If your site does not appear in the top few results, you have a significant penalty. Again indicating its time for the clean-up and Reconsideration Request. From there, if your site isn't performing well, its difficult to differentiate a penalty from other factors that reduce your rankings. It hardly matters. It still means you need to make sure your site is in compliance with Google's Guidelines and that you work on adding fresh, original content to your site and getting links from other well-ranked sites whose main topic is related to yours.
Thanks for your comments rainborick, rep added My site appears fine with your two suggestions but could it be possible that a certain section of a website could get a penalty? If you do site:www.examplesite.com/webpage.php, and it doesn't appear could this just mean its not cached?
My previous commentator is very much right. Please also read http://www.mediadonis.net/?p=378 I tried it myself - but could not related. Still, grapevine is that - this German SEO expert got the theory from a Google staff. So worth the try.
also keep in mind if your page is not there it could be a glitch (temporary) ..it happened to me.....
Or you could just get blocked y google trying to use the site: operator with some bull**** message about spyware on your pc - which is actually google just stopping you seeing what has or has not been indexed.
Do a search like this one: site:yoursite.com If your sites is not showed and was before, you can start thinking abouit being penalizaed.
The most common penalties are the -30 penalty and the -950 penalty (or end of results penalty). Just google for the penalty names and you'll find information on how to see if your site has the penalty and what you can do to clean it up.
Not sure if this was mentioned, but if you are set up in GWT and you are penalized, they will send you a notification email. So if you are being penalized, clean it up and then file an inclusion request w/Google here: http://www.google.com/support/webmasters/bin/answer.py?answer=35843 (sorry, not allowed to post live links yet)