How do you compare items in your inventory to what you have equipped?
You will have to compare the items manually. This feature does not exist in any automated form in Diablo II.
To find the ranges of stats on items, you will need to use a source outside of the game such as The Arreat Summit.
In Diablo III however, it is possible to compare items in an automated fashion.
If you are using the popular third party Slash Diablo servers and modified client, they offer and allow their own Maphack which shows stat ranges.