Thank you for your post.
There are several posts on this forum regarding using a CNS Barcode with FileMaker Go. You may want to visit the following link as it walks you through setting up the barcode scanner with FileMaker Go:
You could create a script after using the barcode scanner to look up the item in a table, and if not found, display a message. The script would have to be created in FileMaker Pro before transferring it to FileMaker Go. The same would apply for importing an Excel file.
Let me know if you need additional clarification.
I received your email.
To create scripts, pull down the Scripts menu and select the first option "Manage Scripts...". In the "Manage Scripts" window that appears, click "New" in the bottom left corner. All of your scripts steps appear in the left column. Double-clcik the script step to have it appear on the right side.
Let me know if you run into any difficulty.