How to look what other players inventory contains?

Provided you are using a multiplayer server, the common (and useful) Essentials plugin comes with an in-build command to view a player's inventory. You can use the /invsee <name> command to achieve this.

You can also modify a users inventory using this command.

In Minecraft Vanilla 1.12.2 you can't look up other player's inventories, you can only test if a certain item is in it. But in 1.13 (and already in the snapshots) you can use /data get entity <player> to get all data of a player. If you add Inventory to it, it only shows the inventory.

To look a player inventory:

/data get entity @p Inventory

     

To look a chest inventory:

/data get block 0 0 0 Items

To set a player inventory in a chest and clean his inventory (command block):

execute at @p run data modify block 762 70 1565 Items set from entity @p Inventory 

clear @p