When using the library catalog to find an item, you may have the message "Ask at the Reference Desk" where a call number should be displayed. This message usually only applies to Federal Document items.
The call number should be listed under the 'details' of the item. You can click the 'More information about this item' bar. Once that expands, you can find the call number in the 'Gov't Docs Number' field.
If for any reason you click on that link and it does not display a call number in that field, contact the reference desk and they can assist you.