Here is an article on the basics of cod liver oil which you might find helpful. It points out the good and the bad.
http://www.westonaprice.org/basicnutrition/codliveroil.html
In this article they refer to Carlson brand, however, they no longer recommend this brand. Here are the current recommended sources as of June 2007 and why.
http://www.westonaprice.org/basicnutrition/clarifications.html
There are many other articles on cod liver oil on the site as well if you want to do some more research. Hope this helps.
If purchased from a good source, I thing you should be fine. Hope this helps.