Not sure what you tried, but the following should work. It assumes your button is CommandButton1
Change the titles of the controls to check as required
Private Sub CommandButton1_Click()
Dim oCC As ContentControl
For Each oCC In ActiveDocument.ContentControls
Select Case oCC.Title
Case "Field 1 Title", "Field 2 Title" 'The titles of the required controls separated by commas
If oCC.ShowingPlaceholderText = True Then
oCC.Range.Select
MsgBox "Complete the field", vbCritical
Exit Sub
End If
Case Else
End Select
Next oCC
'rest of submit button code goes here
End Sub