It used to be that when you typed [[ in VisualEditor, it showed the "wikitext detected" message. Now, it opens the link dialog directly (which is even better); the warning is still shown when you type things like ~~~~.
However, when you copy-paste text that contains [[ or ~~~~, nothing happens, which can lead to user error.
I'm not sure if it's possible (or desirable) to open the link dialog since there might be several wikilinks in the pasted text. Another possibility would be to convert wikilinks automatically. In any case, in the meantime, we should at least display the "wikitext detected" message to warn the user about their mistake.