You could use a user defined function that returns a true/false result if the
cell has a formula.
Option Explicit
Function CellHasFormula(rng As Range) As Boolean
CellHasFormula = rng(1).HasFormula
End Function
Then use that in the format|conditional formatting formula:
If you're new to macros, you may want to read David McRitchie's intro at:
http://www.mvps.org/dmcritchie/excel/getstarted.htm
But I have a followup suggestion.
If you lock those cells with formulas (format|cells|protection tab), then
protect the worksheet (tools|protection|protect sheet), then you don't have to
worry about the user writing over the formula.
(Unlock the cells that that should allow user input, though.)
And a suggestion if you have to allow either the formula or the typing...
Use a couple of helper cells.
A1 can hold the formula
B1 can hold the superseding value
C1 can hold a formula to determine which to use:
=if(b1="",a1,b1)
Then use C1 in all your other calculations (and still lock all those formula
cells.)