You might get a clue by going to File > Info > Edit Links (on the right)
Other wise you may need to use some vba code
If you don't know how try here
www.pptalchemy.co.uk/vba.html
Code:
'Code
Sub findLinks()
Dim oshp As Shape
Dim osld As Slide
For Each osld In ActivePresentation.Slides
For Each oshp In osld.Shapes
If oshp.Type = msoLinkedPicture Then
With osld.Shapes.AddShape(msoShape12pointStar, Left:=oshp.Left, Top:=oshp.Top, Width:=20, Height:=20)
.Fill.ForeColor.RGB = vbRed
End With
End If
If oshp.Type = msoPlaceholder Then
If oshp.PlaceholderFormat.ContainedType = msoLinkedPicture Then
With osld.Shapes.AddShape(msoShape12pointStar, Left:=oshp.Left, Top:=oshp.Top, Width:=20, Height:=20)
.Fill.ForeColor.RGB = vbRed
End With
End If
End If
Next oshp
Next osld
End Sub