How do you compare items in Diablo 2?

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.

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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.

One way to quickly see how item A and item B affect your basic total stats is to equip them back and forth while the character stats screen is open. You'll see how your total damage, dexterity, strength, vitality, stamina and resistances change depending on which item is equipped.