How to know if my site has been penalized by google?

Hi
My site was showing on 4th page for the relevant keywords search on google. And it was there since last 4 days and stayed there. But today, when I checked, it is not there at all in the searches.
I purchased backlinks about a week ago. At that time, I was not aware that google prohibits doing that. Can there be a possibility that my site has been penalized? Is there any way to confirm that? If yes, how can correct this situation?
Please help.
Thanks and regards
Vaibhav

See original post by v4vaibhav