The description of the Robe of Useful Items (BR 195) states that the wearer can detach a patch causing it to become the object or creature it represents.
Does the wearer know which patch contains what object or creature?
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The Robe of Useful Items states (my emphasis):
While wearing the robe, you can use an action to detach one of the patches, causing it to become the object or creature it represents.
This implies that there is a visual representation of the item on the patch - making it clear what the item is to anyone that can see it.
This is also supported by the picture in the DMG: