In Excel 2000, if I put 9 characters in a cell (i.e. "abcdefghi") that
are formatted as "Times New Roman", then format the middle 3 as
"WingDings" (i.e. "def" formatted), then run the following test code:

Public Sub Test()
Dim rngCell As Range
Dim lngIndex As Long

Set rngCell = ActiveCell

For lngIndex = 1 To Len(rngCell.Value)
Debug.Print lngIndex, rngCell.Characters(lngIndex).Font.Name
Next lngIndex
End Sub

I get the following results in the Immediate window:

1 Null
2 Null
3 Null
4 Null
5 Null
6 Null
7 Times New Roman
8 Times New Roman
9 Times New Roman

This is only partially correct, as characters 1-3 should also be "Times
New Roman". Maybe newer versions of Excel have improved the Characters
object. If it would work on your system, then you might be able to use
it to step through each character to find out whether the font had been
changed to some other font.
--
Regards,
Bill Renaud