Maybe you say it is smiple to check out that a site is punished by google. If google a site get less result, or no page from a site, or the site get lower rank, then we see the site is punished by google. But I have puzzled nowaday. Google a site can get more than 200,000 results, and reach more day by day.At the same time, visitors is less and less.I think the site is punished by google, It is very disappointed.Some day the visitors quantities come back from google with a surprise.I'm happy a week till visitors disappear unexpectedly.This is very strange for I do noting during that week.And more strange thing is that three weeks after, visitors quantities from google become big, and also just happy less than a week Is this google dance? But the results just become more and more.Is the site punised by google?But why google would come back once more?I have no idea.
Try to search in google "domainname" (without "www" and domain zone). If you don't see your domain at first place - domain gets google filter. It works if your domain does not consist of popular keywords.
Check the source of traffic. If your site gets less traffic from google slowly, it shows you are being outperformed by your competitors for a particular key word. If the traffic is nil from google, the writing on the wall is clear. You have been filtered out of the G search