First THANKS to lishid for his MARVELOUS plugin OpenInv : OpenInventory.
Without this plugin neither the idea, concept or coding this feature would have been possible. Then THANKS for his hard work.
With the command /openinv (or /oi) (and the right node, see Player Commands) you can open every player's inventory, and play with it.
The Inventory will be open as a chest where you'll be able to interact with it.
BE CAREFUL the 5 lasts slots are not used, if you put an item in it, they will be deleted ! (see the images)
You want to check if a user is not doing something wrong after a new inscription. You can then follow him with invisibility and check everything he take in chest with OpenInv. You can also do some kind of funny things (see above)