If I were you I would take some time and study the different depression glass patterns as Ironlord suggested. True depression glass has a distinctly different feel than modern glass and you will probably see warps and bubbles in it. This is because it was manually pressed into a mold. Here's a topic from a while back with some links and also some detailed pictures of my depression glass mixing bowls, you can really see the imperfections in these because they are bigger, they are on the second page of this topic and you can see an example of the Hazel Atlas mark on the first page:
http://www.antique-shop.com/forums/index.php?topic=6983.0When you've identified a few pieces, spend some time holding them and examining them up close. Get a feel for what old glass feels like and that will make your job much easier. It will also give you a way to connect with something your mother loved.
I think from there you may develop favorite pieces you will want to keep and some you can live without. Not only colors and patterns that you like, but ones that have special memories attached to them. You might also consider gifting a few things to family members who would cherish them as belonging to your mother. Best wishes, I know what you are going through is terribly hard.
