You can check if your site has been affected by looking at each of the known update dates (moz.com/google-algorithm-change) and see if they correspond to a traffic drop on your site. If you had a traffic drop following a known Panda update, then you may have been hit by Panda, which is to do with on-site quality. Again, if the drop was dramatic and followed a known Penguin update, then you are likely hit by Penguin, which affects certain keywords. So if you only have certain keywords that are declining, then this is probably not a Panda problem. But it important to remember that there are many other reasons that can cause traffic drops, such as: a competitor site has outranked you, a web redesign, hosting problems, crawling issues etc. Google could simply lower your rankings and it might not align with any known update.