Identifying Lego kits from unopened bags for library
If you really want to know, I would suggest identifying the most unusual part in a bag, then  using that part, search for what sets it has appeared in recently.  Each piece usually has a small number printed on it.  Use this part number on bricklink, or peeron to search for the part, and then click the link that shows you what sets it has appeared in.

Alternatively, take a picture of the bag and post it to the AFOLs of Facebook group and you'll probably have an answer in 5 minutes or less.