2. Check indexation by inquiry [site:yoursite.com] in Google — whether the number of pages has changed sharply.
3. Check up, whether there are no problems with a site in Google Webmaster Tools.
4. Pay attention on Google PR (the grey strip is the extremely bad sign).
5. Make search in "name-domain" (without a zone, i.e. trend-web.blogspot, for example) — if there is no among the first pages, almost for certain a foul shot.
6. Check up, whether considers Google your site "dangerous":
http://www.google.com/safebrowsing/diagnostic?site=trend-web.blogspot.com
7. Check up HTML a code of pages a site about presence there "another's" links or other alien inserts.
8. Check up other your sites. Happens so, that Google gives a foul shot to the domain on which NS are placed.
9. Make search in Google on exact occurrence of a long phrase from any page of a site — that page about which you a phrase took, should be on the first place.
10. Check up, whether is not present such, that site pages are shown only at parametre addition &filter=0 in url search or in pressing on “show omitted results” (usually similar sanction is imposed for a double-content).