Hello Jeff,
There is no direct way of searching an IDOC is cloud for system, as Cloud for system only receives XML messages . Here is what you need to do (I am assuming in WE21 on your ERP system the content type is set to Text/XML and SOAP protocol option is checked )
In HCI message monitor based on the time stamp locate the message which corresponds to the IDOC. In the message monitor log the IDOC no should be listed in a line similar to "com.sap.sod.utils.idoc.soap.idocassign= [0000000001472565,0000000001472565]" and you should see another line for the message ID similar to "com.sap.sod.utils.idoc.soap.messageid= 005056B239651ED480D7B2C829F3878D"
Take the message id from HCI message monitor and then in the web service monitoring in your Cloud system under the advanced section provide the message id to locate the message.
This is bit cumbersome process specifically the HCI monitoring part but as of now this is only way of searching.
Best regards, Abinash