Normally if we see a message in SAPGUI and we can just double click the message icon to get its technical detail like message id and number displayed.
However in some case the icon is not available for click, for example below, there is a popup window displayed and you could not double click the icon. If you close the popup, the message disappears as well.
In this case, use tcode SE84 to launch repository information system, specify Short Description with value: *cannot be used as proxy object
Then you get answer: message id DDLS, value 444.
Or if you already know that the table T100 stores message text, you can directly query on table T100 to get the same result.