Three add-ons answer "is this better than what I'm wearing" in different ways.
Simple Item Levels
Simple Item Levels writes the item level on item icons everywhere: bags, bank, character sheet, inspect window. It answers "which of these is higher" at a glance.
Pawn
Pawn scores items with stat weights for your class and spec and shows an upgrade arrow in the tooltip. Forever's itemisation is its own, so treat the default weights as a starting point.
RatingBuster
RatingBuster breaks an item's stats into what they mean for your character and compares them with what you have equipped, line by line in the tooltip.
Which to use
Simple Item Levels for the quick look, plus one of the other two for decisions. Pawn if you want a single verdict, RatingBuster if you want the details. All three have Forever builds; their pages show versions and dates.