Getting Penalized By Google (Panda/Penguin)

I'm still trying to figure out what happened to my site's rankings about a year ago, but I don't think it was due to bad linking - at least I don't think it was. I never got a notification from Google for suspicious link building, and I only had about 500 links on a domain that I had owned for about 4 years. So I guess my questions are:

1) Will Google let you know if they downrank you due to "bad" link building?

2) How many backlinks is suspicious? Are we talking hundreds, or thousands, or tens of thousands?

My site was hit by recent Google Panda (January 2013) update and results was a huge drop in traffic. I think that's the biggest sign that you are being hit.