The easiest and quickest approach is to download the C# SDK for MWS, where you can create a client and call the ListInboundShipmentItems method directly. For this particular operation, ShipmentId is required. All you need to do is add your AWS keys and token (if applicable) and make the call. I noticed that the SDK used to be publicly available, but I think now you are required to log in with a seller account.
If you are not using the SDK, your request should look like this:
http://mws.amazonaws.com/FulfillmentInboundShipment/2010-10-01/
?Action=ListInboundShipmentItems
&Version=2010-10-01
&AWSAccessKeyId=1QZHP81EXAMPLEN5R44N
&MWSAuthToken=amzn.mws.4ea38b7b-f563-7709-4bae-87aeaEXAMPLE
&SignatureVersion=2
&SignatureMethod=HmacSHA256
&Signature=VY6sqvdk01VeEXAMPLEG0Vh4oj3
&Timestamp=2015-12-01T02:40:36Z
&SellerId=1234567890
&ShipmentId=SSF85DGIZZ3OF1
Make sure you read the developer guide too.