If an asset has been shared you will see an icon in the Shared with other users column in the Asset Store. The icon will identify how many times you have shared the item.  To see who it has been shared with you can click on this icon or on the info icon to the far right of the asset in your Asset Store which will open the Info Panel for the asset.  This contains information about all shares and you can edit or remove the shares from here.  You can also open the asset and click on the info icon at the top right side of the asset to open the Info Panel.