The Upgrade Finder is generally the best way to quickly estimate whether an item that you don’t have will be an upgrade or not. We have almost all end-game relevant gear available via the various searches, including Delve items.
The “Quick Search” option would probably be what you want, and ensure the Delve filter is set to the level that you want. You can also specify whether or not you want to spend crests to upgrade those items with the “Max Upgrade Threshold” setting.
There is also the “Add to my Bag” search, which lets you manually choose a specific item and rank it as if you owned it.
Note that sometime tomorrow we are updating for the Midnight pre-patch, and we will temporarily disable the Upgrade Finder until Midnight is launched. The reason that we do this is because the game balance gets so wacky during these pre-patches it is hard to give good rankings. You can read more here: World of Warcraft Midnight Pre-Patch