OK

If you add you shape like this:

Sub addTitre()
Dim sld As Slide
Dim shpTitre As Shape
On Error Resume Next
Set sld = ActiveWindow.View.Slide
If Not sld Is Nothing Then
Set shpTitre = sld.Shapes.AddTextbox(msoTextOrientationHorizontal, 36, 21.62504, 648, 90)
shpTitre.Tags.Add "TYPE", "TITRE"
End If
End Sub
The added shape will have an invisible tag

You can check for the shape on any slide using a function:

Function IsTitre(sld As Slide) As Boolean
Dim oshp As Shape
For Each oshp In sld.Shapes
If oshp.Tags("TYPE") = "TITRE" Then
IsTitre = True
Exit For
End If
Next oshp
End Function
To check whether titre is present on a particular slide

Msgbox IsTitre(sld) 'where sld is a reference to the slide